Don't crash on "gemc help" with no topic

The early help handler read argv[i + 1] without a bounds check. For the
bare "gemc help" (the natural way to ask for the help index), i + 1 ==
argc, so argv[argc] (the null terminator) was passed to the std::string
parameter of printOptionOrSwitchHelp, throwing std::logic_error and
aborting. Bounds-check i + 1 < argc and fall back to printHelp() (which
already exists and exits) when no topic follows.

Verified: "gemc help" now prints the help index and exits 0 instead of
terminating with an uncaught exception (Geant4 11.4.1 dev container).

Fixes #121

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: zhaozhiwen

gemc/goptions/goptions.cc

@@ -100,7 +100,9 @@ GOptions::GOptions(int argc, char* argv[], const GOptions& user_defined_options)
 			exit(EXIT_SUCCESS);
 		}
 		else if (strcmp(argv[i], "help") == 0) {
-			printOptionOrSwitchHelp(argv[i + 1]);
+			// "gemc help <topic>" shows topic help; bare "gemc help" shows the general help index.
+			if (i + 1 < argc) { printOptionOrSwitchHelp(argv[i + 1]); }
+			else { printHelp(); }
 			exit(EXIT_SUCCESS);
 		}
 	}