Add ECOFF Symbolic Header sanity checks

Anti-fuzzer measures.  The checks don't ensure the various elements in
the header are distinct, but that isn't important as far as making
sure we don't overrun the buffer containing all the elements.  Also,
we now don't care about offsets where the corresponding count is zero.

	* ecoff.c (_bfd_ecoff_slurp_symbolic_info): Sanity check offsets
	in debug->symbolic_header.