Fix wrong exit code being returned

(cherry picked from commit 0d3a168a532349d0759a033b04efe04e5ef72c32)

drivers/unix/os_unix.cpp

@@ -357,7 +357,7 @@ Error OS_Unix::execute(const String &p_path, const List<String> &p_arguments, bo
 		int status;
 		waitpid(pid, &status, 0);
 		if (r_exitcode)
-			*r_exitcode = WEXITSTATUS(status);
+			*r_exitcode = WIFEXITED(status) ? WEXITSTATUS(status) : status;
 
 	} else {