patch 9.1.0192: drop: don't rewind when editing the same file

Problem:  :drop tries to :rewind the argumentlist, which results in E37
          (after v9.1.0046)
Solution: instead of calling ex_rewind(), call open_buffer() only when
          re-using the initial empty buffer

fixes: #14219
closes: #14220

Signed-off-by: Christian Brabandt <cb@256bit.org>

src/ex_cmds.c

@@ -5503,7 +5503,8 @@ ex_drop(exarg_T *eap)
 		buf_check_timestamp(curbuf, FALSE);
 		curbuf->b_p_ar = save_ar;
 	    }
-	    ex_rewind(eap);
+	    if (buf->b_ml.ml_flags & ML_EMPTY)
+		open_buffer(FALSE, eap, 0);
 	    return;
 	}
     }