CryptoPkg: RuntimeCryptLib: support realloc(NULL, size)

The ISO C standard says about realloc(),

  If ptr is a null pointer, the realloc function behaves like the malloc
  function for the specified size.

The realloc() implementation doesn't conform to this currently, so add a
check and call malloc() if appropriate.

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>

CryptoPkg/Library/BaseCryptLib/SysCall/RuntimeMemAllocation.c

@@ -396,6 +396,10 @@ void *realloc (void *ptr, size_t size)
   UINTN  StartPageIndex;
   UINTN  PageCount;
 
+  if (ptr == NULL) {
+    return malloc (size);
+  }
+
   //
   // Get Original Size of ptr
   //