PR19323 memory allocation greater than 4G

PR19323 memory allocation greater than 4G

On 32-bit targets, memory requested for program/section headers on a
fuzzed binary can wrap to 0.  A bfd_alloc of zero bytes actually
returns a one byte allocation rather than a NULL pointer.  This then
leads to buffer overflows.

Making this check unconditional triggers an extremely annoying gcc-5
warning.

	PR 19323
	* elfcode.h (elf_object_p): Check for ridiculous e_shnum and
	e_phnum values.