zebra: Allow fpm_listener to continue to try to read

Currently when the fpm_listener attempts to read say X
bytes it may only get Y( which is less than X ).  In this
case we should assume that the dplane_fpm_nl code is just
being slow, as that we know it is possible for it to send
a partial fpm message.  Let's just loosen the constraints
a bit and allow data to flow.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>

zebra/fpm_listener.c

@@ -189,7 +189,7 @@ read_fpm_msg(char *buf, size_t buf_len)
 			fprintf(stderr,
 				"Read %lu bytes but expected to read %lu bytes instead\n",
 				bytes_read, need_len);
-			return NULL;
+			continue;
 		}
 
 		if (reading_full_msg)