Silence warning from clang

Silence warning from clang

The original intent was most likely to memset() the entire MD5Context to
zero at the end of MD5Final(), which we do now.

In file included from tools/qcryptographichash.cpp:49:
...qtbase/src/corelib/../../include/QtCore/../../src/corelib/tools/../../3rdparty/md5/md5.cpp:139:24:
warning:
      argument to 'sizeof' in 'memset' call is the same expression as
the
      destination; did you mean to dereference it? [-Wsizeof-pointer-
        memset(ctx, 0, sizeof(ctx));    /* In case it's sensitive */
               ~~~            ^~~

Change-Id: I793c6f0944b89c0e4c5f9253cdb1071175c17152
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>