CryptoPkg: RuntimeCryptLib: support free(NULL)

CryptoPkg: RuntimeCryptLib: support free(NULL)

The ISO C standard says about free(),

  If ptr is a null pointer, no action occurs.

This is not true of the RuntimeFreeMem() internal function. Therefore we
must not forward the argument of free() to RuntimeFreeMem() without
checking.

Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Qin Long <qin.long@intel.com>
Cc: Ting Ye <ting.ye@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Qin Long <qin.long@intel.com>