git_revwalk_push_range: do not crash if range is missing

If someone passes just one ref (i.e. "master") and misses passing the
range we should be nice and return an error code instead of crashing.

src/revwalk.c

@@ -194,6 +194,12 @@ int git_revwalk_push_range(git_revwalk *walk, const char *range)
 	if ((error = git_revparse(&revspec, walk->repo, range)))
 		return error;
 
+	if (!revspec.to) {
+		git_error_set(GIT_ERROR_INVALID, "invalid revspec. Only range supported.");
+		error = GIT_EINVALIDSPEC;
+		goto out;
+	}
+
 	if (revspec.flags & GIT_REVPARSE_MERGE_BASE) {
 		/* TODO: support "<commit>...<commit>" */
 		git_error_set(GIT_ERROR_INVALID, "symmetric differences not implemented in revwalk");

tests/revwalk/basic.c

@@ -400,6 +400,15 @@ void test_revwalk_basic__push_range(void)
 	cl_git_pass(test_walk_only(_walk, commit_sorting_segment, 2));
 }
 
+void test_revwalk_basic__push_range_no_range(void)
+{
+	revwalk_basic_setup_walk(NULL);
+
+	git_revwalk_reset(_walk);
+	git_revwalk_sorting(_walk, 0);
+	cl_git_fail_with(GIT_EINVALIDSPEC, git_revwalk_push_range(_walk, "HEAD"));
+}
+
 void test_revwalk_basic__push_mixed(void)
 {
 	git_oid oid;