- Made CRYPTO_MDEBUG even less used in crypto.h, giving
   MemCheck_start() and MemCheck_stop() only one possible definition.
 - Made the values of the debug function pointers in mem.c dependent
   on the existence of the CRYPTO_MDEBUG macro, and made the rest of
   the code understand the NULL case.

That's it.  With this code, the old behvior of the debug functionality
is restored, but you can still opt to have it on, even when the
library wasn't compiled with a defined CRYPTO_MDEBUG.