rtsp: Fix parsing of IPV6 addresses in RTSP links

Fix the colon position pointer (used for finding the port) to
account for the brackets that are removed from the IPV6 address
to avoid an invalid string pointer. Additionally, ensure that the
port, if present, is passed back to ParseURL to be processed to
avoid an assertion.

CRs-Fixed: 804202
Change-Id: I0f874b6e22795f016a3621d3800b0bcde07cb401