io_posix.cc:1377 error: comparison of integers of different signs: 'long long' and 'unsigned long long'

### Expected behavior

The build runs without any warning.

### Actual behavior

The following warning is shown (and handled as an error by default) :

```
io_posix.cc:1377:39: error: comparison of integers of different signs: 'long long' and 'unsigned long long' [-Werror,-Wsign-compare]
```

### Steps to reproduce the behavior

```
mkdir build
cd build
cmake .. -DANDROID_PLATFORM=android-24 -DANDROID_ABI=armeabi-v7a -DANDROID_STL=c++_shared -DCMAKE_TOOLCHAIN_FILE=/opt/android-ndk-linux/build/cmake/android.toolchain.cmake
```

Reproduce the warning with the provided CMake Android configuration, then inspect io_posix.cc at line 1377 and the surrounding comparison. Verify the change by rebuilding with the same configuration; done means the signedness warning no longer appears or fails the build.

Written by the indexing model from the issue text.