rtsp: deal with borked server responses

- enforce a response body length of 0, if the
  response has no Content-lenght. This is according
  to the RTSP spec.
- excess bytes in a response body are forwarded to
  the client writers which will report and fail the
  transfer

Follow-up to d7b6ce6
Fixes #12701
Closes #12706

tests/data/Makefile.inc

@@ -96,7 +96,7 @@ test644 test645 test646 test647 test648 test649 test650 test651 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 \
+test681 test682 test683 test684 test685 test686 test687 test688 test689 \
 \
 test700 test701 test702 test703 test704 test705 test706 test707 test708 \
 test709 test710 test711 test712 test713 test714 test715 test716 test717 \

tests/data/test689

@@ -0,0 +1,53 @@
+<testcase>
+
+#Informational
+<info>
+<keywords>
+RTSP
+OPTIONS
+</keywords>
+</info>
+
+# Server-side
+<reply>
+<data>
+RTSP/7.1 786          
+
+RTSP/          
+</data>
+<datacheck>
+</datacheck>
+</reply>
+
+# Client-Side
+<client>
+<server>
+rtsp
+</server>
+<tool>
+lib567
+</tool>
+
+<name>
+fuzzing crash issue #12701
+</name>
+<command>
+rtsp://%HOSTIP:%RTSPPORT/%TESTNUMBER
+</command>
+</client>
+
+<verify>
+<protocol>
+OPTIONS rtsp://%HOSTIP:%RTSPPORT/%TESTNUMBER RTSP/1.0
+CSeq: 1
+User-Agent: test567
+Test-Number: 567
+
+</protocol>
+# 8 == CURLE_WEIRD_SERVER_REPLY
+<errorcode>
+8
+</errorcode>
+</verify>
+
+</testcase>