This fixes ubsan errors:

ext/pb_ds/detail/cc_hash_table_map_/cc_ht_map_.hpp:533:15: runtime error: member access within null pointer of type 'struct entry'

libstdc++-v3/ChangeLog:

	* include/ext/pb_ds/detail/cc_hash_table_map_/cc_ht_map_.hpp
	(find_key_pointer(key_const_reference, false_type))
	(find_key_pointer(key_const_reference, true_type)): Do not
	dereference null pointer.

Tested x86_64-linux. Committed to trunk.