Fix some issues in b_print.c code

Convert assert to OPENSSL_assert(), add some documentation, add the calls
to fmtfp() for the "e" and "g" floating point formats which were missing.

Based on a patch provided by Ger Hobbelt <ger@hobbelt.com>.

RT#2270

Reviewed-by: Richard Levitte <levitte@openssl.org>