patch 8.2.0911: crash when opening a buffer for the cmdline window fails

Problem:    Crash when opening a buffer for the cmdline window fails. (Chris
            Barber)
Solution:   Check do_ecmd() succeeds.  Reset got_int if "q" was used at the
            more prompt. (closes #6211)
diff --git a/src/testdir/dumps/Test_cmdwin_interrupted.dump b/src/testdir/dumps/Test_cmdwin_interrupted.dump
new file mode 100644
index 0000000..325bc54
--- /dev/null
+++ b/src/testdir/dumps/Test_cmdwin_interrupted.dump
@@ -0,0 +1,18 @@
+> +0&#ffffff0@74
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|d+0#0000000&|o|n|e| @52|0|,|0|-|1| @8|A|l@1|