I stumbled over this problem while chasing something else.  If the
current source symtab belongs to a loaded shared library, and the
objfile for that library is freed in the process or rerunning the
executable, the current source symtab can be left pointing to garbage.

An alternative solution, though a little more heavy handed, would be
to replace the previous clear_pc_function_cache() call with a call to
clear_symtab_users().

Comments?

-Fred