bio/b_addr.c: resolve HP-UX compiler warnings.

The warning reads "[cast] may cause misaligned access". Even though
this can be application-supplied pointer, misaligned access shouldn't
happen, because structure type is "encoded" into data itself, and
application would customarily pass correctly aligned pointer. But
there is no harm in resolving the warning...

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5894)