[unknown] frames when profilee quits before the profiler

It is reproducible with `offwaketime` and `offcputime` scripts. Other scripts I didn't test.

## Steps to reproduce *(in terms of terminal commands)*

```haskell
$ cat test.c
#include

void f() {
for (size_t i = 0; i < 5; ++i) {
sleep(1);
}
}

void g() {
f();
}

int main() {
g();
}
$ gcc test.c -o test -O0 -g3
$ ./test &; sudo /usr/share/bcc/tools/offwaketime -Up $(pgrep test)
[1] 127746
Tracing blocked time (us) by user off-CPU and waker stack... Hit Ctrl-C to end.
[1] + 127746 done ./test
^C
waker: swapper/3 0
-- --
b'[unknown]'
target: test 127746
3000441

```

### Expected

Stack something like

```haskell
waker: swapper/3 0
-- --
b'[f]'
b'[g]'
b'[main]'
target: test 127746
3000441
```

### Actual

Vast part of the stack is missing, and the one frame from userspace says `unknown` which doesn't match any actual function name.

## Version

* bcc: tested both with `0.13.0` and `0.14.0`
* kernel: tested with `5.4.28` and `5.6.4`