qemu-ga: Fix a SIGSEGV in ga_run_command() helper

qemu-ga on a NetBSD -current VM terminates with a SIGSEGV upon receiving
'guest-set-time' command...

Core was generated by `qemu-ga'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x000000000cd37a40 in ga_pipe_read_str (fd=fd@entry=0xffffff922a20, str=str@entry=0xffffff922a18)
    at ../qga/commands-posix.c:88
88	        *str[len] = '\0';
[Current thread is 1 (process 1112)]
(gdb) bt
#0  0x000000000cd37a40 in ga_pipe_read_str (fd=fd@entry=0xffffff922a20, str=str@entry=0xffffff922a18)
    at ../qga/commands-posix.c:88
#1  0x000000000cd37b60 in ga_run_command (argv=argv@entry=0xffffff922a90,
    action=action@entry=0xcda34b8 "set hardware clock to system time", errp=errp@entry=0xffffff922a70, in_str=0x0)
    at ../qga/commands-posix.c:164
#2  0x000000000cd380c4 in qmp_guest_set_time (has_time=<optimized out>, time_ns=<optimized out>,
    errp=errp@entry=0xffffff922ad0) at ../qga/commands-posix.c:304
#3  0x000000000cd253d8 in qmp_marshal_guest_set_time (args=<optimized out>, ret=<optimized out>, errp=0xffffff922b48)
    at qga/qga-qapi-commands.c:193
#4  0x000000000cd4e71c in qmp_dispatch (cmds=cmds@entry=0xcdf5b18 <ga_commands>, request=request@entry=0xf3c711a4b000,
    allow_oob=allow_oob@entry=false, cur_mon=cur_mon@entry=0x0) at ../qapi/qmp-dispatch.c:220
#5  0x000000000cd36524 in process_event (opaque=0xf3c711a79000, obj=0xf3c711a4b000, err=0x0) at ../qga/main.c:677
#6  0x000000000cd526f0 in json_message_process_token (lexer=lexer@entry=0xf3c711a79018, input=0xf3c712072480,
    type=type@entry=JSON_RCURLY, x=28, y=1) at ../qobject/json-streamer.c:99
#7  0x000000000cd93860 in json_lexer_feed_char (lexer=lexer@entry=0xf3c711a79018, ch=125 '}', flush=flush@entry=false)
    at ../qobject/json-lexer.c:313
#8  0x000000000cd93a00 in json_lexer_feed (lexer=lexer@entry=0xf3c711a79018,
    buffer=buffer@entry=0xffffff922d10 "{\"execute\":\"guest-set-time\"}\n", size=<optimized out>)
    at ../qobject/json-lexer.c:350
#9  0x000000000cd5290c in json_message_parser_feed (parser=parser@entry=0xf3c711a79000,
    buffer=buffer@entry=0xffffff922d10 "{\"execute\":\"guest-set-time\"}\n", size=<optimized out>)
    at ../qobject/json-streamer.c:121
#10 0x000000000cd361fc in channel_event_cb (condition=<optimized out>, data=0xf3c711a79000) at ../qga/main.c:703
#11 0x000000000cd3710c in ga_channel_client_event (channel=<optimized out>, condition=<optimized out>, data=0xf3c711b2d300)
    at ../qga/channel-posix.c:94
#12 0x0000f3c7120d9bec in g_main_dispatch () from /usr/pkg/lib/libglib-2.0.so.0
#13 0x0000f3c7120dd25c in g_main_context_iterate_unlocked.constprop () from /usr/pkg/lib/libglib-2.0.so.0
#14 0x0000f3c7120ddbf0 in g_main_loop_run () from /usr/pkg/lib/libglib-2.0.so.0
#15 0x000000000cda00d8 in run_agent_once (s=0xf3c711a79000) at ../qga/main.c:1522
#16 run_agent (s=0xf3c711a79000) at ../qga/main.c:1559
#17 main (argc=<optimized out>, argv=<optimized out>) at ../qga/main.c:1671
(gdb)

The commandline options used on the host machine...
qemu-system-aarch64 \
   -machine type=virt,pflash0=rom \
   -m 8G \
   -cpu host \
   -smp 8 \
   -accel hvf \
   -device virtio-net-pci,netdev=unet \
   -device virtio-blk-pci,drive=hd \
   -drive file=netbsd.qcow2,if=none,id=hd \
   -netdev user,id=unet,hostfwd=tcp::2223-:22 \
   -object rng-random,filename=/dev/urandom,id=viornd0 \
   -device virtio-rng-pci,rng=viornd0 \
   -serial mon:stdio \
   -display none \
   -blockdev node-name=rom,driver=file,filename=/opt/homebrew/Cellar/qemu/9.0.2/share/qemu/edk2-aarch64-code.fd,read-only=true \
   -chardev socket,path=/tmp/qga_netbsd.sock,server=on,wait=off,id=qga0 \
   -device virtio-serial \
   -device virtconsole,chardev=qga0,name=org.qemu.guest_agent.0

This patch rectifies the operator precedence while assigning the NUL
terminator.

Fixes: c3f32c13a3

Signed-off-by: Sunil Nimmagadda <sunil@nimmagadda.net>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Link: https://lore.kernel.org/r/m15xppk9qg.fsf@nimmagadda.net
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>

qga/commands-posix.c

@@ -84,7 +84,7 @@ static ssize_t ga_pipe_read_str(int fd[2], char **str)
         *str = g_realloc(*str, len + n + 1);
         memcpy(*str + len, buf, n);
         len += n;
-        *str[len] = '\0';
+        (*str)[len] = '\0';
     }
     close(fd[0]);
     fd[0] = -1;