easy: make curl_easy_perform() return error if connection still there

This typically happens if CURL_CONNECT_ONLY is used and a second
curl_easy_perform() is attempted.

A connection "taken over" with CURL_CONNECT_ONLY cannot be ended any
other way than a curl_easy_cleanup() on the easy handle that holds it.

Add test 696 to verify.

Closes #16003

tests/data/Makefile.am

@@ -101,7 +101,7 @@ test652 test653 test654 test655 test656 test658 test659 test660 test661 \
 test662 test663 test664 test665 test666 test667 test668 test669 test670 \
 test671 test672 test673 test674 test675 test676 test677 test678 test679 \
 test680 test681 test682 test683 test684 test685 test686 test687 test688 \
-test689 test690 test691 test692 test693 test694 test695 \
+test689 test690 test691 test692 test693 test694 test695 test696 \
 \
 test700 test701 test702 test703 test704 test705 test706 test707 test708 \
 test709 test710 test711 test712 test713 test714 test715 test716 test717 \

tests/data/test696

@@ -0,0 +1,52 @@
+<testcase>
+<info>
+<keywords>
+HTTP
+HTTP GET
+</keywords>
+</info>
+
+<reply>
+<data>
+HTTP/1.1 200 OK swsclose
+Server: test-server/fake
+Last-Modified: Tue, 13 Jun 2000 12:10:00 GMT
+Content-Length: 6
+Connection: close
+
+-foo-
+</data>
+</reply>
+
+#
+# Client-side
+<client>
+<server>
+http
+</server>
+<tool>
+lib%TESTNUMBER
+</tool>
+<name>
+CONNECT_ONLY and doing a second curl_easy_perform
+</name>
+<command>
+http://%HOSTIP:%HTTPPORT
+</command>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+<protocol>
+GET /556 HTTP/1.1
+Host: ninja
+
+</protocol>
+
+# 43 == CURLE_BAD_FUNCTION_ARGUMENT
+<errorcode>
+43
+</errorcode>
+</verify>
+</testcase>