Fix a few Cppcheck issues

Nothing fancy, just a couple of tweaks.

Remaining issues

```
[ast.hpp:783]: (performance) Function parameter 'disp' should be passed by reference.
[context.hpp:62]: (style) The class 'Data' does not have a constructor.
[emscripten_wrapper.cpp:22]: (error) Uninitialized struct member: options.precision
[environment.hpp:28]: (performance) Function parameter 'key' should be passed by reference.
[environment.hpp:35]: (performance) Function parameter 'key' should be passed by reference.
[environment.hpp:44]: (performance) Function parameter 'key' should be passed by reference.
[environment.hpp:57]: (performance) Function parameter 'key' should be passed by reference.
[source_map.cpp:12]: (warning) Member variable 'SourceMap::base64vlq' is not initialized in the constructor.
[sass2scss\sass2scss.cpp:315]: (performance) Function parameter 'sass' should be passed by reference.
[sass2scss\sass2scss.h:50]: (performance) Function parameter 'sass' should be passed by reference.
```