[llvm-readobj] - --gnu-hash-table: do not crash when the GNU hash table goes past the EOF.

We might have a scenario where a the `GbuHashTable` variable correctly points
to a place inside the file (we validate this fact early in `parseDynamicTable`),
but nbuckets/maskwords fields are broken in the way the code tries
to read the data past the EOF. This patch fixes the issue.

Differential revision: https://reviews.llvm.org/D79853