value check was looking at the code

Marlin/gcode.h

@@ -135,7 +135,7 @@ public:
     static bool seen(const char c) {
       char *p = strchr(command_args, c);
       const bool b = !!p;
-      if (b) value_ptr = DECIMAL_SIGNED(*p) ? p + 1 : NULL;
+      if (b) value_ptr = DECIMAL_SIGNED(*(p + 1)) ? p + 1 : NULL;
       return b;
     }