guard against nullptr

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67899 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/osx/core/bitmap.cpp

@@ -1639,6 +1639,8 @@ bool wxMask::Create(const wxBitmap& bitmap)
     unsigned char * destdatabase = (unsigned char*) m_memBuf.GetWriteBuf( size ) ;
     wxASSERT( destdatabase != NULL ) ;
     
+    if ( destdatabase )
+    {
         memset( destdatabase , 0 , size ) ;
         unsigned char * srcdata = (unsigned char*) bitmap.GetRawAccess() ;
 
@@ -1660,6 +1662,7 @@ bool wxMask::Create(const wxBitmap& bitmap)
                     *destdata++ = 0x00 ;
             }
         }
+    }
 
     m_memBuf.UngetWriteBuf( size ) ;
     RealizeNative() ;