stackcount return *blank* stack.

I run the command below in my Android device:
```
python stackcount -p 22119 /apex/com.android.runtime/lib64/bionic/libc.so:memset
```

There're many records without any stack information.
I think even it is using FP to unwind the stack, at least the first frame stack with 'memset' should be shown. Any idea on this issue? Thanks in advance!
```
b'memset'
b'[unknown]'
310

311

322

433

b'memset'
b'MterpInvokeVirtual'
b'mterp_op_invoke_virtual'
b'art::interpreter::Execute(art::Thread*, art::CodeItemDataAccessor const&, art::ShadowFrame&, art::JValue, bool, bool) (.llvm.8678274965593772940)'
b'artQuickToInterpreterBridge'
b'art_quick_to_interpreter_bridge'
564

b'memset'
b'[unknown]'
652

1059

1205

17162

31876
```

Begin with the stackcount entry point and reproduce the reported invocation against /apex/com.android.runtime/lib64/bionic/libc.so:memset on Android, comparing records containing only [unknown] with complete samples. Trace the unwinding path and verify the expected first-frame behavior; done means the blank-stack cases are explained and the intended output is tested.

Written by the indexing model from the issue text.