Fix read-of-uninitialized introduced in r253277 exposed on some buildbots

Verified that this was at least /an/ issue, if not the only one, by
initializing NumBuckets to 1 (previously it was uninitialized, so if
this change made a difference, which it did (causing a bunch of tests to
crash) it demonstrates use-of-uninitialized memory). Initializing then
removes the crashes.

Thanks Reid for the debugging assistance

llvm-svn: 253395