Fix crash in struct-with-sig-2.exp with debug-names target board

When I run the struct-with-sig-2.exp test with the .debug_names-using
target board, I see a gdb crash.  This happens because the reader
throws an exception without calling finalize_all_units.  This causes
an assertion failure later because the number of CUs and TUs doesn't
match.

The fix is to clear 'all_units' on failure.

Approved-By: Tom de Vries <tdevries@suse.de>