make room for the .dpu suffix

dpu.c

@@ -23,7 +23,7 @@ int main (int argc, char **argv){
 		fputs("I pity the fool who don't have an input file!\n",stderr);
 		return EXIT_FAILURE;
 	}
-	if(strlen(argv[arg]) >= FILENAME_MAX)
+	if(strlen(argv[arg]) >= FILENAME_MAX - 4)
 	{
 		fputs("It's a filename, not a novel.\n", stderr);
 		return EXIT_FAILURE;