main/app: Only look to end of file if ':end' is specified, and not just ':'

There is a little known feature in app_controlplayback that will cause the
specified offset to be used relative to the end of a file if a ':end' is
detected within the filename.

This feature is pretty bad, but okay.

However, a bug exists in this code where a ':' detected in the filename
will cause the end pointer to be non-NULL, even if the full ':end' isn't
specified. This causes us to treat an unspecified offset (0) as being
"start playing from the end of the file", resulting in no file playback
occurring.

This patch fixes this bug by resetting the end pointer if ':end' is not
found in the filename.

Change-Id: Ib4c7b1b45283e4effd622a970055c51146892f35