Fix a memory leak occuring whenever win_res() is called

[stevenyvr987]

win_res() is called multiple times when sxiv initializes, whenever an Xresource value is needed. For each call, win_res() creates an Xresources database from a specified window display.

1. First, it calls XrmInitialize() to initialize the Xresources manager
2. then calls XResourceManagerString() to get a pointer to the Xrm
3. then calls XrmGetStringDatabase() with the pointer to get a pointer to a new Xresources database
4. and finally, calls XrmGetResource() with the database pointer to get the XrmValue.

The leak occurs because win_res() stores the database pointer as an automatic and does not call XrmDestroyDatabase() before the pointer goes out of scope. The obvious fix is to call XrmDestroyDatabase() before returning. But this is not performant; since win_res() is called multiple times, we should avoid creating and destroying a database for each call.

The proposed fix is to have win_res() store the Xresources manager pointer and the database pointer as local statics. win_res() calls XrmInitialize(), XResourceManagerString(), and XrmGetStringDatabase() once, if the Xresources manager pointer has never been set. Thereafter, win_res() calls XrmGetResource() with the static database pointer for whatever value is needed. The end result is the database is created once and all memory will be collected when sxiv quits.

If XResourceManagerString() returns a null pointer, win_res() will continue to try to create a database for each call, but this will not be worse than the current situation.

If XrmGetStringDatabase() returns a null pointer, win_res() will return the given default value, as before, but a ?: operator is used instead of an if-else statement.

[xyb3rt]

Thanks for reporting and fixing this. I used a simpler solution that does not use static variables.