ld: Avoid overflows in string merging

ld: Avoid overflows in string merging

as the bug report shows we had an overflow in the test if
hash table resizing is needed.  Reorder the expression to avoid
that.  There's still a bug somewhere in gracefully handling
failure in resizing (e.g. out of memory), but this pushes the
boundary for that occurring somewhen into the future and
immediately helps the reporter.

    bfd/

    PR ld/31009
    * merge.c (NEEDS_RESIZE): New macro avoiding overflow.
    (sec_merge_maybe_resize): Use it.
    (sec_merge_hash_insert): Ditto.