fix OOB fwrite

.gitignore

@@ -0,0 +1,4 @@
+bits-*
+perf.*
+*.svg
+obj/

src/bits.c

@@ -20,7 +20,7 @@ static int fwrite_all(const void* buf, size_t esz, size_t num, FILE* out)
 	register size_t w, done=0;
 	register size_t fsz = esz * num;
 	const char* tbuf = (const char*)buf;
-	while( (w = fwrite(tbuf+done, 1, fsz, out)) > 0 && done < fsz) done+=w;
+	while( (w = fwrite(tbuf+done, 1, fsz-done, out)) > 0 && done < fsz) done+=w;
 	if (done!=fsz) return 0;
 	else return 1;
 }