session: fix use after free of session->devs as reported by valgrind

session: fix use after free of session->devs as reported by valgrind

==7478== Invalid write of size 8
==7478==    at 0x4E59182: sr_session_dev_remove_all (session.c:302)
==7478==    by 0x4E591CD: sr_session_destroy (session.c:265)
==7478==    by 0x4095D9: load_input_file_module (input.c:143)
==7478==    by 0x4097AB: load_input_file (input.c:157)
==7478==    by 0x40531E: main (main.c:288)
==7478==  Address 0x7877eb8 is 88 bytes inside a block of size 96 free'd
==7478==    at 0x4C2A37C: free (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==7478==    by 0x4E5F454: sr_input_free (input.c:573)
==7478==    by 0x4095C3: load_input_file_module (input.c:140)
==7478==    by 0x4097AB: load_input_file (input.c:157)
==7478==    by 0x40531E: main (main.c:288)