Fix memory leak in high level API (#781)

Fix memory leak in high level API (#781)

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.c`s `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.