smart_pkt: check whether error packets are prefixed with "ERR "

In the `git_pkt_parse_line` function, we determine what kind of packet
a given packet line contains by simply checking for the prefix of that
line. Except for "ERR" packets, we always only check for the immediate
identifier without the trailing space (e.g. we check for an "ACK"
prefix, not for "ACK "). But for "ERR" packets, we do in fact include
the trailing space in our check. This is not really much of a problem at
all, but it is inconsistent with all the other packet types and thus
causes confusion when the `err_pkt` function just immediately skips the
space without checking whether it overflows the line buffer.

Adjust the check in `git_pkt_parse_line` to not include the trailing
space and instead move it into `err_pkt` for consistency.

(cherry picked from commit 786426ea)

src/transports/smart_pkt.c

@@ -107,10 +107,12 @@ static int comment_pkt(git_pkt **out, const char *line, size_t len)
 
 static int err_pkt(git_pkt **out, const char *line, size_t len)
 {
-	git_pkt_err *pkt;
+	git_pkt_err *pkt = NULL;
 	size_t alloclen;
 
 	/* Remove "ERR " from the line */
+	if (git__prefixncmp(line, len, "ERR "))
+		goto out_err;
 	line += 4;
 	len -= 4;
 
@@ -127,6 +129,11 @@ static int err_pkt(git_pkt **out, const char *line, size_t len)
 	*out = (git_pkt *) pkt;
 
 	return 0;
+
+out_err:
+	giterr_set(GITERR_NET, "error parsing ERR pkt-line");
+	git__free(pkt);
+	return -1;
 }
 
 static int data_pkt(git_pkt **out, const char *line, size_t len)
@@ -463,7 +470,7 @@ int git_pkt_parse_line(
 		ret = ack_pkt(head, line, len);
 	else if (!git__prefixncmp(line, len, "NAK"))
 		ret = nak_pkt(head);
-	else if (!git__prefixncmp(line, len, "ERR "))
+	else if (!git__prefixncmp(line, len, "ERR"))
 		ret = err_pkt(head, line, len);
 	else if (*line == '#')
 		ret = comment_pkt(head, line, len);