Another Coverity "fix"...

This is kinda valid... I am reading a 32-bit integer from a socket,
then trying to loop that many times.  What Coverity didn't notice
though is that the loop will exit way before due to the buffer
being exhausted.

By ensuring that it's less than UINT32_MAX/4, I'm still not really
enforcing anything since with the 32-bit value for extcnt, and a
32-bit value for every string, there could only be strings before
it failed anyway, even with a 4GB buffer (which you couldn't do).

src/sftp/sftp_attr.c

@@ -321,6 +321,17 @@ sftp_getfattr(sftp_rx_pkt_t pkt)
 	if (ret->flags & SSH_FILEXFER_ATTR_EXTENDED) {
 		uint32_t extcnt = sftp_get32(pkt);
 		uint32_t ext;
+		/*
+		 * This is to silence Coverity...
+		 * Coverity knows extcnt is tainted, and
+		 * so I "should" range-check it before using
+		 * it to control loop iterations.
+		 * This loop is actually controlled by the
+		 * size of the buffer since sftp_getstring()
+		 * will fail long before we reach extcnt if
+		 * it has a maliciously high value.
+		 */
+		extcnt &= 0x3FFFFFFF;
 		for (ext = 0; ext < extcnt; ext++) {
 			sftp_str_t type = sftp_getstring(pkt);
 			if (type == NULL)