Fix crash in TinyTagEditor::runAction() with files without extensions - #652

When a user opens the Tiny Tag Editor for a song without a file extension and selects the "Filename" option (option 20), ncmpcpp crashes.

The issue occurs in the code that splits the filename into a base name and extension:

size_t dot = filename.rfind(".");
std::string extension = filename.substr(dot);  // CRASH if dot == npos
filename = filename.substr(0, dot);             // OK - npos returns whole string

If the filename doesn't contain a '.', rfind(".") returns std::string::npos, and calling substr(npos) throws std::out_of_range, causing the crash. This affects files without extensions and stream URIs.

The fix checks if dot != std::string::npos before attempting to extract the extension:

std::string extension;
if (dot != std::string::npos)
{
    extension = filename.substr(dot);
    filename = filename.substr(0, dot);
}