builtin/f_match: prevent overruns of input_string

If these values do not match exactly, it is because the UTF-8 is invalid
anyway and we counted codepoints differently than oniguruma did.  Perhaps it
would be better to error out here, but at least one similar loop already
uses < vs != and since we're off the rails anyway this might be OK.  It is
certainly better than overruning the buffer.

Resolves https://github.com/stedolan/jq/issues/1804

src/builtin.c

@@ -865,7 +865,7 @@ static jv f_match(jq_state *jq, jv input, jv regex, jv modifiers, jv testmode) {
       if (region->end[0] == region->beg[0]) {
         unsigned long idx;
         const char *fr = (const char*)input_string;
-        for (idx = 0; fr != input_string+region->beg[0]; idx++) {
+        for (idx = 0; fr < input_string+region->beg[0]; idx++) {
           fr += jvp_utf8_decode_length(*fr);
         }
         jv match = jv_object_set(jv_object(), jv_string("offset"), jv_number(idx));
@@ -902,7 +902,7 @@ static jv f_match(jq_state *jq, jv input, jv regex, jv modifiers, jv testmode) {
             cap = jv_object_set(cap, jv_string("string"), jv_null());
           } else {
             fr = input_string;
-            for (idx = 0; fr != input_string+region->beg[i]; idx++) {
+            for (idx = 0; fr < input_string+region->beg[i]; idx++) {
               fr += jvp_utf8_decode_length(*fr);
             }
             cap = jv_object_set(jv_object(), jv_string("offset"), jv_number(idx));
@@ -914,7 +914,7 @@ static jv f_match(jq_state *jq, jv input, jv regex, jv modifiers, jv testmode) {
           continue;
         }
         fr = input_string;
-        for (idx = len = 0; fr != input_string+region->end[i]; len++) {
+        for (idx = len = 0; fr < input_string+region->end[i]; len++) {
           if (fr == input_string+region->beg[i]) idx = len, len=0;
           fr += jvp_utf8_decode_length(*fr);
         }