Fix crash in dwarf2read.c with template parameters

Fix crash in dwarf2read.c with template parameters

PR c++/24470 concerns a crash in dwarf2read.c that occurs with a
particular test case.

The issue turns out to be that process_structure_scope will pass NULL
to symbol_symtab.  This happens because new_symbol decided not to
create a symbol for the particular DIE.

This patch fixes the problem by finding another reasonably-appropriate
symtab to use instead; issuing a complaint if one cannot be found for
some reason.

As mentioned in the bug, I think there are other bugs here.  For
example, when using "ptype" on the "l" object in the test case, I
think I would expect to see the template parameter.  I didn't research
this too closely, since it seemed more important to fix the crash.

Tested on x86-64 Fedora 29.

I'd like to check this in to the 8.3 branch as well.

2019-04-30  Tom Tromey  <tromey@adacore.com>

	PR c++/24470:
	* dwarf2read.c (process_structure_scope): Handle case where type
	has template parameters but no symbol was created.

gdb/testsuite/ChangeLog
2019-04-30  Tom Tromey  <tromey@adacore.com>

	PR c++/24470:
	* gdb.cp/temargs.cc: Add test code from PR.