aspeed/soc: Fix possible divide by zero

Coverity reports a possible DIVIDE_BY_ZERO issue regarding the
"ram_size" object property. This can not happen because RAM has
predefined valid sizes per SoC. Nevertheless, add a test to
close the issue.

Fixes: Coverity CID 1547113
Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
[ clg: Rewrote commit log ]
Signed-off-by: Cédric Le Goater <clg@redhat.com>

hw/arm/aspeed_ast27x0.c

@@ -211,6 +211,8 @@ static void aspeed_ram_capacity_write(void *opaque, hwaddr addr, uint64_t data,
     ram_size = object_property_get_uint(OBJECT(&s->sdmc), "ram-size",
                                         &error_abort);
 
+    assert(ram_size > 0);
+
     /*
      * Emulate ddr capacity hardware behavior.
      * If writes the data to the address which is beyond the ram size,