libdatasource: fix null dereference

mediaHTTP can be NULL when failed to make http connection
from http service, and then mediaHTTP->connect() will crash.
check if mediaHTTP is NULL before calling mediaHTTP->connect().

Bug: 186381982
Change-Id: I936baf9fbbb2001f8f48f0f1c15f6bc08513600a
Test: http streaming ut