cookies: only save the cookie file if the engine is enabled

Follow-up to 8eddb8f425.

If the cookieinfo pointer is NULL there really is nothing to save.

Without this fix, we got a problem when a handle was using shared object
with cookies and is told to "FLUSH" it to file (which worked) and then
the share object was removed and when the easy handle was closed just
afterwards it has no cookieinfo and no cookies so it decided to save an
empty jar (overwriting the file just flushed).

Test 1905 now verifies that this works.

Assisted-by: Michael Wallner
Assisted-by: Marcel Raad

Closes #3621

tests/data/test1905

@@ -0,0 +1,60 @@
+<testcase>
+<info>
+<keywords>
+HTTP
+cookies
+FLUSH
+</keywords>
+</info>
+
+# Server-side
+<reply>
+<data nocheck="yes">
+HTTP/1.1 200 OK
+Date: Thu, 09 Nov 2010 14:49:00 GMT
+Server: test-server/fake
+Content-Type: text/html
+Funny-head: yesyes swsclose
+Set-Cookie: foobar=name;
+Set-Cookie: secondcookie=present;
+
+</data>
+</reply>
+
+# Client-side
+<client>
+<server>
+http
+</server>
+ <name>
+CURLOPT_COOKIELIST set to "FLUSH" of a shared cookie object
+ </name>
+<tool>
+lib1905
+</tool
+<command>
+http://%HOSTIP:%HTTPPORT/we/want/1905
+</command>
+</client>
+
+# Verify data after the test has been "shot"
+<verify>
+<strip>
+^User-Agent:.*
+</strip>
+<protocol>
+GET /we/want/1905 HTTP/1.1
+Host: %HOSTIP:%HTTPPORT
+Accept: */*
+
+</protocol>
+<file name="log/cookies1905" mode="text">
+# Netscape HTTP Cookie File
+# https://curl.haxx.se/docs/http-cookies.html
+# This file was generated by libcurl! Edit at your own risk.
+
+%HOSTIP	FALSE	/we/want/	FALSE	0	secondcookie	present
+%HOSTIP	FALSE	/we/want/	FALSE	0	foobar	name
+</file>
+</verify>
+</testcase>