support unpack from ctype pointers

Author: andreax79

cstruct/mem_cstruct.py

@@ -73,6 +73,9 @@ def unpack_from(self, buffer: Optional[bytes], offset: int = 0, flexible_array_l
             self.__mem__ = ctypes.create_string_buffer(self.size + 1)
         elif isinstance(buffer, ctypes.Array):
             self.__mem__ = buffer
+        elif isinstance(buffer, int):
+            # buffer is a pointer
+            self.__mem__ = ctypes.cast(buffer, ctypes.POINTER(ctypes.c_char * self.size)).contents
         else:
             self.__mem__ = ctypes.create_string_buffer(buffer)
         for field, field_type in self.__fields_types__.items():