libdw: Make sure Ebl is always freed from cfi frame cache.

libdw: Make sure Ebl is always freed from cfi frame cache.

libdwfl sets the Dwfl_Module Ebl for the eh_cfi and dwarf_cfi cache to
save a bit of memory. It also calls ebl_closebackend on the ebl to free
it. The Dwarf_CFI never frees the Ebl in the cache, even when it opened
one itself. This means that if only libdw calls are used to access the
Dwarf_CFI the Ebl might be leaked.

Always destroy the Dwarf_CFI cache Ebl in __libdw_destroy_frame_cache.
And in __libdwfl_module_free clear the Dwarf_CFI Ebl if it is the
Dwfl_Module Ebl before calling dwarf_cfi_end and dwarf_end.

Signed-off-by: Mark Wielaard <mjw@redhat.com>