Fix potential null pointer access during system program execution

For the first child process execution, `TWG(process)` is `NULL`; we
need to catch that to avoid undefined behavior.

Author: cmb69

TSRM/tsrm_win32.c

@@ -374,13 +374,13 @@ static process_pair *process_get(FILE *stream)
 	process_pair *ptr;
 	process_pair *newptr;
 
-	for (ptr = TWG(process); ptr < (TWG(process) + TWG(process_size)); ptr++) {
+	for (ptr = TWG(process); ptr && ptr < (TWG(process) + TWG(process_size)); ptr++) {
 		if (ptr->stream == stream) {
 			break;
 		}
 	}
 
-	if (ptr < (TWG(process) + TWG(process_size))) {
+	if (ptr && ptr < (TWG(process) + TWG(process_size))) {
 		return ptr;
 	}