smart_pkt: fix "ng" parser accepting non-space character

When parsing "ng" packets, we blindly assume that the character
immediately following the "ng" prefix is a space and skip it. As the
calling function doesn't make sure that this is the case, we can thus
end up blindly accepting an invalid packet line.

Fix the issue by using `git__prefixncmp`, checking whether the line
starts with "ng ".

(cherry picked from commit b5ba7af2)

src/transports/smart_pkt.c

@@ -306,9 +306,9 @@ static int ng_pkt(git_pkt **out, const char *line, size_t len)
 
 	eol = line + len;
 
-	if (len < 3)
+	if (git__prefixncmp(line, len, "ng "))
 		goto out_err;
-	line += 3; /* skip "ng " */
+	line += 3;
 
 	if (!(ptr = memchr(line, ' ', eol - line)))
 		goto out_err;