som: buffer overflow writing strings

som: buffer overflow writing strings

Code in som_write_symbol_strings neglected to allow for padding, which
can result in a buffer overflow.  It also used xrealloc, which we're
not supposed to use in libbfd because libbfd isn't supposed to call
exit.  Also a realloc is perhaps not a good idea when none of the
buffer contents are needed, so replace with free, bfd_malloc.  There
were three copies of the string handling code, so rather than fix them
all I've extracted them to a function.  This necessitated making one
of the fields in struct som_symbol unsigned.

	* som.c (add_string): New function.
	(som_write_space_strings, som_write_symbol_strings): Use it.
	* som.h (som_symbol_type <stringtab_offset>): Make unsigned.