osmux: fix nullpointer dereference

in point_lookup() the connection pointer is determined using
mgcp_conn_get_rtp() this function may return 0. At the moment
there are no nullpointer checks implemented

Add checks to test for nullpointer.

Fixes: Coverity CID#178662
Change-Id: If9a3c1ac002bc8adc90ca1c1c3dd1db4feea07ac
1 file changed