libssh2_packet_read() was checking the "errno" even when recv() didn't

return an error.  "errno" should have been zero by the recv() call but
doesn't seem to be on all systems.  Only check when recv() says there
is an error.

Bug:		1703467
Patch:		1703468

src/transport.c

@@ -312,7 +312,7 @@ libssh2pack_t libssh2_packet_read(LIBSSH2_SESSION *session)
                 /* check if this is due to EAGAIN and return
                    the special return code if so, error out
                    normally otherwise */
-                if(errno == EAGAIN) {
+                if ((nread < 0) && (errno == EAGAIN)) {
                     return PACKET_EAGAIN;
                 }
                 return PACKET_FAIL;