Fixed possible crashes (segfault).

 * Added check if op_ctx->parent is correct in function
   "axis2_msg_ctx_get_op_ctx".
 * Removed check if op_ctx->parent is set in function
   "axis2_msg_ctx_set_op_ctx". Sometimes parent is set, but
   contains incorrect pointer.
 * Fixed check of environment reference counter. Sometimes it
   is less than 0. (It ought not to be, but it is.)
   Often crash happens when ref < 0. That is why now it must
   equal 0 when we free the memory. Unfortunately this may
   cause memory leaks.