udp_recv: Fix clang warning.

clang does not seem to like adding to a string literal:

	udp_recv.c:51:45: warning: adding 'size_t' (aka 'unsigned long') to a string does not append to the string [-Wstring-plus-int]
		if (memcmp(iov[1].iov_base, FEC_EOF_PACKET + iov[0].iov_len,
					    ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~
	udp_recv.c:51:45: note: use array indexing to silence this warning

That's kind of silly, but if this simple change makes clang happy,
so be it.