launch: appliance: Fix parsing of QEMU_OPTIONS.

If there was no quoting character, the options could run off the end
of the string and into whatever string which happened to be following
in memory.

This bug was revealed when libguestfs was compiled on arm.

src/launch-appliance.c

@@ -136,8 +136,12 @@ add_cmdline_shell_unquoted (guestfs_h *g, const char *options)
       endp = options + strlen (options);
     }
 
-    if (quote == ' ')
-      nextp = endp+1;
+    if (quote == ' ') {
+      if (endp[0] == '\0')
+        nextp = endp;
+      else
+        nextp = endp+1;
+    }
     else {
       if (!endp[1])
         nextp = endp+1;