[OpenMP] libomp: properly initialize buckets in __kmp_dephash_extend

The buckets are initialized in __kmp_dephash_create but when they are extended
the memory is allocated but not NULL'd, potentially leaving some buckets
uninitialized after all entries have been copied into the new allocation.
This commit makes sure the buckets are properly initialized with NULL before
copying the entries.

Differential Revision: https://reviews.llvm.org/D95167