PR29677, Field `the_bfd` of `asymbol` is uninitialised

Besides not initialising the_bfd of synthetic symbols, counting
symbols when sizing didn't match symbols created if there were any
dynsyms named "".  We don't want synthetic symbols without names
anyway, so get rid of them.  Also, simplify and correct sanity checks.

	PR 29677
	* mach-o.c (bfd_mach_o_get_synthetic_symtab): Rewrite.