Fix macros that call macros crashing (#21383)

When macros call macros, the null termination should remain in place until the macro is completed, and only then put back. Otherwise, the macro handler interprets this as setting the macro called to a new value.

Marlin/src/gcode/gcode.cpp

     char * const delim = strchr(gcode, '\n');         // Get address of next newline
     if (delim) *delim = '\0';                         // Replace with nul
     parser.parse(gcode);                              // Parse the current command
-    if (delim) *delim = '\n';                         // Put back the newline
     process_parsed_command(true);                     // Process it
+    if (delim) *delim = '\n';                         // Put back the newline
     if (!delim) break;                                // Last command?
     gcode = delim + 1;                                // Get the next command
   }