Lex: Check whether the header map buffer has space for the buckets

Check up front whether the header map buffer has space for all of its
declared buckets.

There was already a check in `getBucket()`, but it had UB (comparing
pointers that were outside of objects in the error path) and was
insufficient (only checking for a single byte of the relevant bucket).
I fixed the check, moved it to `checkHeader()`, and left a fixed version
behind as an assertion.

llvm-svn: 261449