Fix Coverity 1498612: integer overflow

Fix Coverity 1498612: integer overflow

The assert added cannot ever fail because (current & 0xFFFF) != 0 from the
while loop and the trailing zero bit count therefore cannot be as large as 32.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/17892)