suspension: prevent entering an invalid byte upon resume (with S-Lang)

For some reason, when returning from suspension, SLang will produce
either a clipped error code (0xFF instead of 0xFFFF, when returning
from an externally induced suspension), or it will clip the code of
first subsequent keystroke to a single byte (when returning from a
normal, in-editor suspension: ^Z).

Side-step this by ignoring the clipped error code, and by using an
undefined control code as the first fake keystroke.  Ignoring the
clipped error code is not possible when using a single-byte locale,
otherwise the user would not be able to type the character with
code 0xFF (although it could still be entered with Esc Esc 255).

This fixes https://savannah.gnu.org/bugs/?51477.

src/nano.c

@@ -1288,7 +1288,7 @@ RETSIGTYPE do_continue(int signal)
     terminal_init();
 #endif
     /* Tickle the input routine so it will update the screen. */
-    ungetch(KEY_F0);
+    ungetch(KEY_FLUSH);
 }
 
 #ifndef NANO_TINY

src/nano.h

@@ -576,6 +576,15 @@ enum
 #define SHIFT_HOME 0x40f
 #define SHIFT_END 0x410
 
+#ifdef USE_SLANG
+#ifdef ENABLE_UTF8
+#define KEY_BAD 0xFF  /* Clipped error code. */
+#endif
+#define KEY_FLUSH 0x91  /* User-definable control code. */
+#else
+#define KEY_FLUSH KEY_F0  /* Nonexistent function key. */
+#endif
+
 #ifndef NANO_TINY
 /* An imaginary key for when we get a SIGWINCH (window resize). */
 #define KEY_WINCH -2

src/winio.c

@@ -704,7 +704,10 @@ int parse_kbinput(WINDOW *win)
 	/* Slang and SunOS 5.7-5.9 don't support KEY_RESIZE. */
 	case KEY_RESIZE:
 #endif
-	case KEY_F0:
+#if defined(USE_SLANG) && defined(ENABLE_UTF8)
+	case KEY_BAD:
+#endif
+	case KEY_FLUSH:
 	    return ERR;
     }