RT4347: Fix GCC unused-value warnings with HOST_c2l()

The HOST_c2l() macro assigns the value to the specified variable, but also
evaluates to the same value. Which we ignore, triggering a warning.

To fix this, just cast it to void like we did in commit 08e55364


("Fix some clang warnings.") for a bunch of other instances.

Signed-off-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Andy Polyakov <appro@openssl.org>