fix(PIProcess): handle fork() failure (return value -1)

fork() can return -1 on error (too many processes, out of memory).
The old code treated pid_ == -1 as a successful fork, entering the
parent branch and calling waitpid(-1, ...) which waits for ANY
child process. Added explicit check: close pipes, free memory,
and return on fork failure.

libs/main/system/piprocess.cpp

@@ -319,6 +319,13 @@ void PIProcess::startProc(bool detached) {
 	auto largs = convertToCharArrays(args);
 	auto lenv  = convertToCharArrays(env);
 	int pid_   = fork();
+	if (pid_ < 0) {
+		piCoutObj << "\"fork\" error: " << errorString();
+		PRIVATE->closeAllPipes();
+		delete[] largs;
+		delete[] lenv;
+		return;
+	}
 	if (!detached) PRIVATE->pid = pid_;
 	if (pid_ == 0) {
 		if (!wd.isEmpty()) {