Fix soft-delete hanging on *nix when the command was not found

When execvp() failed to replace the stack, the forked process would
return to Blender's WM_main(..) loop then hang when attempting to draw
the window.

Resolve by calling _exit() when execvp fails.

source/blender/blenlib/intern/fileops_c.cc

@@ -1253,10 +1253,15 @@ static int delete_soft(const char *filepath, const char **error_message)
     return 0;
   }
 
-  execvp(args[0], (char **)args);
+  const int status = execvp(args[0], (char **)args);
 
-  *error_message = "Forking process failed.";
-  return -1; /* This should only be reached if `execvp` fails and stack isn't replaced. */
+  /* This should only be reached if `execvp` fails and stack isn't replaced. */
+  /* Use `_exit` instead of `exit` so Blender's `atexit` cleanup functions don't run. */
+  _exit(status);
+
+  BLI_assert_unreachable();
+
+  return -1;
 }
 #  endif