crtlib: fix undefined behaviour when stripping extenstion from empty string

public/crtlib.c

@@ -858,7 +858,11 @@ void COM_StripExtension( char *path )
 {
 	size_t	length;
 
-	length = Q_strlen( path ) - 1;
+	length = Q_strlen( path );
+
+	if( length > 0 )
+		length--;
+
 	while( length > 0 && path[length] != '.' )
 	{
 		length--;