pingpong: return error when trying to send without connection

When imap_done() got called before a connection is setup, it would try
to "finish up" and dereffed a NULL pointer.

Test case 1553 managed to reproduce. I had to actually use a host name
to try to resolve to slow it down, as using the normal local server IP
will make libcurl get a connection in the first curl_multi_perform()
loop and then the bug doesn't trigger.

Fixes #1953
Assisted-by: Max Dymond

tests/data/Makefile.inc

@@ -169,7 +169,7 @@ test1520 test1521 \
 test1525 test1526 test1527 test1528 test1529 test1530 test1531 test1532 \
 test1533 test1534 test1535 test1536 test1537 test1538 \
 test1540 \
-test1550 test1551 test1552 \
+test1550 test1551 test1552 test1553 \
 test1600 test1601 test1602 test1603 test1604 test1605 test1606 \
 \
 test1700 test1701 test1702 \

tests/data/test1553

@@ -0,0 +1,52 @@
+<testcase>
+<info>
+<keywords>
+IMAP
+Clear Text
+FETCH
+</keywords>
+</info>
+
+#
+# Server-side
+<reply>
+<data>
+From: me@somewhere
+To: fake@nowhere
+
+body
+
+--
+  yours sincerely
+</data>
+<datacheck>
+</datacheck>
+<servercmd>
+</servercmd>
+</reply>
+
+#
+# Client-side
+<client>
+<server>
+imap
+</server>
+ <name>
+IMAP cleanup before a connection was created
+ </name>
+# tool is what to use instead of 'curl'
+<tool>
+lib1553
+</tool>
+ <command>
+imap://non-existing-host.haxx.se:%IMAPPORT/1553
+</command>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+<protocol>
+</protocol>
+</verify>
+</testcase>