ref-filter: allow NULL filter pattern

When setting up `struct ref_filter` for filter_refs(), the
`name_patterns` field must point to an array of pattern strings even if
no patterns are required. To improve this interface, treat a NULL
`name_patterns` field the same as when it points to an empty array.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jltobler

ref-filter.c

@@ -2664,7 +2664,7 @@ static int match_name_as_path(const char **pattern, const char *refname,
 /* Return 1 if the refname matches one of the patterns, otherwise 0. */
 static int filter_pattern_match(struct ref_filter *filter, const char *refname)
 {
-	if (!*filter->name_patterns)
+	if (!filter->name_patterns || !*filter->name_patterns)
 		return 1; /* No pattern always matches */
 	if (filter->match_as_path)
 		return match_name_as_path(filter->name_patterns, refname,
@@ -2751,7 +2751,7 @@ static int for_each_fullref_in_pattern(struct ref_filter *filter,
 		return for_each_fullref_with_seek(filter, cb, cb_data, 0);
 	}
 
-	if (!filter->name_patterns[0]) {
+	if (!filter->name_patterns || !filter->name_patterns[0]) {
 		/* no patterns; we have to look at everything */
 		return for_each_fullref_with_seek(filter, cb, cb_data, 0);
 	}