gdb: handle INTERNALVAR_FUNCTION in clear_internalvar

While checking the list of leaks reported by ASan, I found that
clear_internalvar doesn't free the internal_function object owned by the
internalvar when the internalvar is of kind INTERNALVAR_FUNCTION, fix
that.

Change-Id: I78f53b83b97bae39370a7b5ba5e1cec70626d66a
Reviewed-By: Guinevere Larsen <guinevere@redhat.com>