Ensure static oom error message not detached

Error messages that are detached are assumed to be dynamically
allocated.  Passing a pointer to the static oom error message
can cause an attempt to free the static buffer later.  This change
checks if the oom error message is about to be detached and detaches
a copy instead.

src/errors.c

@@ -125,10 +125,14 @@ int giterr_detach(git_error *cpy)
 	if (!error)
 		return -1;
 
-	cpy->message = error->message;
+	if (error == &g_git_oom_error) {
+		cpy->message = git__strdup(error->message);
+	} else {
+		cpy->message = error->message;
+		error->message = NULL;
+	}
 	cpy->klass = error->klass;
 
-	error->message = NULL;
 	giterr_clear();
 
 	return 0;