gdb/block: add assert in block_iterator::compunit_symtab

gdb/block: add assert in block_iterator::compunit_symtab

Verify that the compunit is only accessed when the iterator is in the
mode where that field makes sense.

We could also set compunit_symtab_ even in "single block" mode, it
wouldn't hurt, but it would not be useful either.

Change-Id: I237db16bb485148f9e906b2e7238159e9f0a8585
Approved-By: Tom Tromey <tom@tromey.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>
Tested-By: Guinevere Larsen <guinevere@redhat.com>