UI: Fix incorrect parameter value

The last parameter is the buffer size (which would include null
terminator), not the desired string length.

UI/obs-app.cpp

@@ -2007,7 +2007,7 @@ bool GetFileSafeName(const char *name, std::string &file)
 		return false;
 
 	wfile.resize(len);
-	os_utf8_to_wcs(name, base_len, &wfile[0], len);
+	os_utf8_to_wcs(name, base_len, &wfile[0], len + 1);
 
 	for (size_t i = wfile.size(); i > 0; i--) {
 		size_t im1 = i - 1;