Tweak to gcode.h

Marlin/gcode.h

     static bool seen(const char c) {
       char *p = strchr(command_args, c);
       const bool b = !!p;
-      if (b) value_ptr = DECIMAL_SIGNED(*(p + 1)) ? p + 1 : NULL;
+      if (b) value_ptr = DECIMAL_SIGNED(p[1]) ? &p[1] : NULL;
       return b;
     }