crypto/engine/eng_list.c: compare getenv rv to NULL instead of 0

Signed-off-by: Patrick Steuer <patrick.steuer@de.ibm.com>

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4958)