pkt-line: fix packet_read_line() to handle len < 0 errors

Update packet_read_line() to test for len > 0 to avoid potential bug
if read functions return lengths less than zero to indicate errors.

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Found/Fixed-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pkt-line.c

@@ -315,7 +315,7 @@ static char *packet_read_line_generic(int fd,
 			      PACKET_READ_CHOMP_NEWLINE);
 	if (dst_len)
 		*dst_len = len;
-	return len ? packet_buffer : NULL;
+	return (len > 0) ? packet_buffer : NULL;
 }
 
 char *packet_read_line(int fd, int *len_p)