smart_pkt: adjust style of "ref" packet parsing function

While the function parsing ref packets doesn't have any immediately
obvious buffer overflows, it's style is different to all the other
parsing functions. Instead of checking buffer length while we go, it
does a check up-front. This causes the code to seem a lot more magical
than it really is due to some magic constants. Refactor the function to
instead make use of the style of other packet parser and verify buffer
lengths as we go.

(cherry picked from commit 5edcf5d1)

src/transports/smart_pkt.c

@@ -206,33 +206,25 @@ static int sideband_error_pkt(git_pkt **out, const char *line, size_t len)
  */
 static int ref_pkt(git_pkt **out, const char *line, size_t len)
 {
-	int error;
 	git_pkt_ref *pkt;
 	size_t alloclen;
 
-	if (len < GIT_OID_HEXSZ + 1) {
-		giterr_set(GITERR_NET, "error parsing pkt-line");
-		return -1;
-	}
-
-	pkt = git__malloc(sizeof(git_pkt_ref));
+	pkt = git__calloc(1, sizeof(git_pkt_ref));
 	GITERR_CHECK_ALLOC(pkt);
-
-	memset(pkt, 0x0, sizeof(git_pkt_ref));
 	pkt->type = GIT_PKT_REF;
-	if ((error = git_oid_fromstr(&pkt->head.oid, line)) < 0)
-		goto error_out;
-
-	/* Check for a bit of consistency */
-	if (line[GIT_OID_HEXSZ] != ' ') {
-		giterr_set(GITERR_NET, "error parsing pkt-line");
-		error = -1;
-		goto error_out;
-	}
 
-	/* Jump from the name */
-	line += GIT_OID_HEXSZ + 1;
-	len -= (GIT_OID_HEXSZ + 1);
+	if (len < GIT_OID_HEXSZ || git_oid_fromstr(&pkt->head.oid, line) < 0)
+		goto out_err;
+	line += GIT_OID_HEXSZ;
+	len -= GIT_OID_HEXSZ;
+
+	if (git__prefixncmp(line, len, " "))
+		goto out_err;
+	line++;
+	len--;
+
+	if (!len)
+		goto out_err;
 
 	if (line[len - 1] == '\n')
 		--len;
@@ -244,16 +236,18 @@ static int ref_pkt(git_pkt **out, const char *line, size_t len)
 	memcpy(pkt->head.name, line, len);
 	pkt->head.name[len] = '\0';
 
-	if (strlen(pkt->head.name) < len) {
+	if (strlen(pkt->head.name) < len)
 		pkt->capabilities = strchr(pkt->head.name, '\0') + 1;
-	}
 
 	*out = (git_pkt *)pkt;
 	return 0;
 
-error_out:
+out_err:
+	giterr_set(GITERR_NET, "error parsing REF pkt-line");
+	if (pkt)
+		git__free(pkt->head.name);
 	git__free(pkt);
-	return error;
+	return -1;
 }
 
 static int ok_pkt(git_pkt **out, const char *line, size_t len)