Continue standardising malloc style for libcrypto

Continuing from previous commit ensure our style is consistent for malloc
return checks.

Reviewed-by: Kurt Roeckx <kurt@openssl.org>

crypto/mem_dbg.c

diff --git a/crypto/mem_dbg.c b/crypto/mem_dbg.c
index 2cd1169..c19847e 100644
--- a/crypto/mem_dbg.c
+++ b/crypto/mem_dbg.c
@@ -792,7 +792,7 @@
     MemCheck_off();
     b = BIO_new(BIO_s_file());
     MemCheck_on();
-    if (!b)
+    if (b == NULL)
         return;
     BIO_set_fp(b, fp, BIO_NOCLOSE);
     CRYPTO_mem_leaks(b);