asan: som: heap-buffer-overflow

asan: som: heap-buffer-overflow

Triggered by overflow of size calulation resulting in a too small
buffer.  The testcase found one of the som_bfd_count_ar_symbols
problems.

	* som.c (setup_sections): Don't overflow space_strings_size.  Use
	bfd_malloc2 to catch overflow of size calculation.
	(som_prep_for_fixups): Use bfd_zalloc2 to catch overflow of size
	calculation.
	(som_build_and_write_symbol_table): Similarly use bfd_zmalloc2.
	(som_slurp_symbol_table): Similarly use bfd_zmalloc2, bfd_malloc2,
	and bfd_zalloc2.
	(bfd_som_attach_aux_hdr): Use size_t vars for string length.
	(som_bfd_count_ar_symbols): Use bfd_malloc2 to catch overflow of
	size calculation.  Use size_t vars for length and catch overflow.
	(som_slurp_armap): Use bfd_alloc2 to catch overflow of size
	calculation.
	(som_bfd_ar_write_symbol_stuff): Similarly use bfd_zmalloc2 and
	bfd_malloc2.  Perform size calculations in bfd_size_type.