patch 8.1.1134: buffer for quickfix window is reused for another file

Problem:    Buffer for quickfix window is reused for another file.
Solution:   Don't reuse the quickfx buffer. (Yegappan Lakshmanan)

src/buffer.c

@@ -1853,6 +1853,7 @@ curbuf_reusable(void)
 	&& curbuf->b_ffname == NULL
 	&& curbuf->b_nwindows <= 1
 	&& (curbuf->b_ml.ml_mfp == NULL || BUFEMPTY())
+	&& !bt_quickfix(curbuf)
 	&& !curbufIsChanged());
 }