Fix memory leak in high level API

[matthiasgoergens]

See #780 for context.

Previously, in the high level API if we received a signal between setting up signal handlers and processing INIT, we would leak

$ ./example/hello -s -d -f mountpoint/
[9/9] Linking target example/hello_ll
FUSE library version: 3.14.1
nullpath_ok: 0

=================================================================
==178330==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 352 byte(s) in 1 object(s) allocated from:
    #0 0x7fbb19abf411 in __interceptor_calloc /usr/src/debug/gcc/gcc/libsanitizer/asan/asan_malloc_linux.cpp:77
    #1 0x7fbb1a0efd3b in fuse_fs_new ../lib/fuse.c:4814
    #2 0x7fbb1a0f02b5 in fuse_new_31 ../lib/fuse.c:4913
    #3 0x7fbb1a10ec5e in fuse_main_real ../lib/helper.c:345
    #4 0x5625db8ab418 in main ../example/hello.c:176
    #5 0x7fbb1983c78f  (/usr/lib/libc.so.6+0x2378f)

SUMMARY: AddressSanitizer: 352 byte(s) leaked in 1 allocation(s).

That's because fuse_lowlevel.cs fuse_session_destroy would only call the user supplied op.destroy, if INIT had been processed, but the high level API relied on op.destroy to free f->fs.

This patch moves the freeing into fuse_destroy that will always be called by our high-level API.

[bsbernd]

Hmm, ok, so if the file system allocated something at startup (and not in fs->op.init) and that would still leak. I wonder what is the reason to call se->op.destroy() only when se->got_init() was run.

[matthiasgoergens]

I wonder what is the reason to call se->op.destroy() only when se->got_init() was run.

I also wondered about this one. But since that is part of the already established, publicly observable behaviour of the low level API, I did not want to change it for this PR.

(I don't know whether it's documented, but it's definitely observable and clients likely rely on this.)

[bsbernd]

Yeah I know, I wonder if we should add a session flag. For sure that is something that could turned around to be the default if there ever should be another incompatible libfuse version.