fix SHA2* cleanups to zero-out the complete context

fix SHA2* cleanups to zero-out the complete context

GCC 4.8 is now clever enough to warn about:

contrib/sha2_internal.cc: In function ‘char* SHA256_End(SHA256_CTX*, char*)’:
contrib/sha2_internal.cc:656:31: warning: argument to ‘sizeof’ in ‘void*
memset(void*, int, size_t)’ call is the same expression as the destination;
did you mean to dereference it? [-Wsizeof-pointer-memaccess]
   MEMSET_BZERO(context, sizeof(context));

So fix it as suggested. Its interesting though that the SHA2*
calculation as far as we need it works even without zeroing out.

Git-Dch: Ignore