check return value
see https://sourceforge.net/p/gpac/code/4720/

git-svn-id: svn://svn.code.sf.net/p/gpac/code/trunk/gpac@4723 63c20433-aa62-49bd-875c-5a186b69a8fb
Merge branch 'plain'

Merge revision: r4723
Conflicts:
	src/media_tools/dash_client.c