patch 8.2.2036: buffer messed up if creating the quickfix window fails

Problem:    Current buffer is messed up if creating a new buffer for the
            quickfix window fails.
Solution:   Check that creating the buffer succeeds. (closes #7352)
diff --git a/src/quickfix.c b/src/quickfix.c
index 40897b6..5e46ad2 100644
--- a/src/quickfix.c
+++ b/src/quickfix.c
@@ -4151,13 +4151,15 @@
     if (qf_buf != NULL)
     {
 	// Use the existing quickfix buffer
-	(void)do_ecmd(qf_buf->b_fnum, NULL, NULL, NULL, ECMD_ONE,
-		ECMD_HIDE + ECMD_OLDBUF, oldwin);
+	if (do_ecmd(qf_buf->b_fnum, NULL, NULL, NULL, ECMD_ONE,
+				      ECMD_HIDE + ECMD_OLDBUF, oldwin) == FAIL)
+	    return FAIL;
     }
     else
     {
 	// Create a new quickfix buffer
-	(void)do_ecmd(0, NULL, NULL, NULL, ECMD_ONE, ECMD_HIDE, oldwin);
+	if (do_ecmd(0, NULL, NULL, NULL, ECMD_ONE, ECMD_HIDE, oldwin) == FAIL)
+	    return FAIL;
 
 	// save the number of the new buffer
 	qi->qf_bufnr = curbuf->b_fnum;