Use scoped_free_pendings in coff_symtab_read

Use scoped_free_pendings in coff_symtab_read

PR gdb/18624 concerns an assertion failure that occurs when setting a
breakpoint in a Go program on Windows.

What happens here is that coff_symtab_read uses buildsym but does not
instantiate scoped_free_pendings.  So, the struct pending objects are
never released.  Later, dwarf2read.c calls buildsym_init, which
asserts.

This patch fixes the problem by instantiating scoped_free_pendings in
coff_symtab_read.

Tested using the test executable from the PR.  I don't know how to
test this more fully.

gdb/ChangeLog
2018-07-17  Tom Tromey  <tom@tromey.com>

	PR gdb/18624:
	* coffread.c (coff_symtab_read): Use scoped_free_pendings.