Populate captures also for zero-width matches

Instead of just using {"captures":[]}.

sub functions are use captures for replacement expressions.
If we don't populate, captures for empty matches, the replacement
expression is run with an empty object as input instead of an object
containing the named captures with "" as value:

* before:

  $ jq -n '"123foo456bar" | gsub("[^a-z]*(?<x>[a-z]*)"; "Z\(.x)")'
  "ZfooZbarZnull"

* after:

  $ jq -n '"123foo456bar" | gsub("[^a-z]*(?<x>[a-z]*)"; "Z\(.x)")'
  "ZfooZbarZ"

---

I also removed a redundant

  result = NULL;
  if (result) {
    ...
  }

src/builtin.c

@@ -931,7 +931,17 @@ static jv f_match(jq_state *jq, jv input, jv regex, jv modifiers, jv testmode) {
         jv match = jv_object_set(jv_object(), jv_string("offset"), jv_number(idx));
         match = jv_object_set(match, jv_string("length"), jv_number(0));
         match = jv_object_set(match, jv_string("string"), jv_string(""));
-        match = jv_object_set(match, jv_string("captures"), jv_array());
+        jv captures = jv_array();
+        for (int i = 1; i < region->num_regs; ++i) {
+          jv cap = jv_object();
+          cap = jv_object_set(cap, jv_string("offset"), jv_number(idx));
+          cap = jv_object_set(cap, jv_string("string"), jv_string(""));
+          cap = jv_object_set(cap, jv_string("length"), jv_number(0));
+          cap = jv_object_set(cap, jv_string("name"), jv_null());
+          captures = jv_array_append(captures, cap);
+        }
+        onig_foreach_name(reg, f_match_name_iter, &captures);
+        match = jv_object_set(match, jv_string("captures"), captures);
         result = jv_array_append(result, match);
         // ensure '"qux" | match("(?=u)"; "g")' matches just once
         start = (const UChar*)(input_string+region->end[0]+1);
@@ -1005,8 +1015,6 @@ static jv f_match(jq_state *jq, jv input, jv regex, jv modifiers, jv testmode) {
   } while (global && start <= end);
   onig_region_free(region,1);
   region = NULL;
-  if (region)
-    onig_region_free(region,1);
   onig_free(reg);
   jv_free(input);
   jv_free(regex);

tests/onig.test

@@ -1,7 +1,7 @@
 # match builtin
 [match("( )*"; "g")]
 "abc"
-[{"offset":0, "length":0, "string":"", "captures":[]}, {"offset":1, "length":0, "string":"", "captures":[]}, {"offset":2, "length":0, "string":"", "captures":[]}, {"offset":3, "length":0, "string":"", "captures":[]}]
+[{"offset":0,"length":0,"string":"","captures":[{"offset":0,"string":"","length":0,"name":null}]},{"offset":1,"length":0,"string":"","captures":[{"offset":1,"string":"","length":0,"name":null}]},{"offset":2,"length":0,"string":"","captures":[{"offset":2,"string":"","length":0,"name":null}]},{"offset":3,"length":0,"string":"","captures":[{"offset":3,"string":"","length":0,"name":null}]}]
 
 [match("( )*"; "gn")]
 "abc"
@@ -145,6 +145,10 @@ gsub("\\b(?<x>.)"; "\(.x|ascii_downcase)")
 "ABC DEF"
 "aBC dEF"
 
+gsub("[^a-z]*(?<x>[a-z]*)"; "Z\(.x)")
+"123foo456bar"
+"ZfooZbarZ"
+
 # utf-8
 sub("(?<x>.)"; "\(.x)!")
 "’"