fix(interp): don't fseek(NULL) on M99 in MDI

src/emc/rs274ngc/interp_convert.cc

@@ -5150,16 +5150,21 @@ int Interp::convert_stop(block_pointer block,    //!< pointer to a block of RS27
     PROGRAM_STOP();
   } else if (block->m_modes[4] == 1) {
     OPTIONAL_PROGRAM_STOP();
-  } else if (block->m_modes[4] == 99 && _setup.loop_on_main_m99) {
+  } else if (block->m_modes[4] == 99 && _setup.loop_on_main_m99 &&
+             settings->file_pointer != NULL) {
 
     // Fanuc-style M99 main program endless loop
+    // Only loops when running from a file; in MDI there is no file to
+    // seek back to, so M99 falls through to the M2/M30 program-end path
+    // below (avoids a fseek(NULL) segfault).
     logDebug("M99 main program endless loop");
 
     loop_to_beginning(settings);  // return control to beginning of file
     FINISH();  // Output any final linked segments
     return INTERP_EXECUTE_FINISH;  // tell task to issue any queued commands
   } else if ((block->m_modes[4] == 2) || (block->m_modes[4] == 30) ||
-            (block->m_modes[4] == 99 && !_setup.loop_on_main_m99)
+            (block->m_modes[4] == 99 &&
+             (!_setup.loop_on_main_m99 || settings->file_pointer == NULL))
             ) {   /* reset stuff here */
 
 /*1*/

src/emc/rs274ngc/interp_o_word.cc

@@ -523,7 +523,8 @@ void Interp::loop_to_beginning(setup_pointer settings)
 	     settings->filename);
 
     // scroll back to beginning of file/first block
-    fseek(settings->file_pointer, 0, SEEK_SET);
+    if (settings->file_pointer != NULL)
+        fseek(settings->file_pointer, 0, SEEK_SET);
     settings->sequence_number = 0;
 }