Fix multiple resource mappings causing a crash

Author: misyltoad

retrace/dxgiretrace.py

@@ -548,6 +548,8 @@ def invokeInterfaceMethod(self, interface, method):
                 print('    else')
                 print('        _pbData = *ppData;')
                 print('    _desc.RefCount++;')
+                print('    if (!ppData)')
+                print('        return;')
             else:
                 print('    _MAP_DESC _MapDesc;')
                 print('    _getMapDesc(_this, %s, _MapDesc);' % ', '.join(method.argNames()))