session: free memory for datafeed callbacks, reported by valgrind

session: free memory for datafeed callbacks, reported by valgrind

==17549== 32 (16 direct, 16 indirect) bytes in 1 blocks are definitely lost in loss record 22 of 39
==17549==    at 0x4C29110: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==17549==    by 0x5359200: g_malloc (in /usr/lib64/libglib-2.0.so.0.4200.2)
==17549==    by 0x536EE2D: g_slice_alloc (in /usr/lib64/libglib-2.0.so.0.4200.2)
==17549==    by 0x5370165: g_slist_append (in /usr/lib64/libglib-2.0.so.0.4200.2)
==17549==    by 0x4E595C3: sr_session_datafeed_callback_add (session.c:512)
==17549==    by 0x409527: load_input_file_module (input.c:111)
==17549==    by 0x4097AB: load_input_file (input.c:157)
==17549==    by 0x40531E: main (main.c:288)