util: Clear unused part of the map in virBitmapShrink

util: Clear unused part of the map in virBitmapShrink

Some of the other functions depend on the fact that unused bits and longs are
always zero and it's less error-prone to clear it than fix the other functions.
It's enough to zero out one piece of the map since we're calling realloc() to
get rid of the rest (and updating map_len).

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1540817

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>