Improve partial local writes

If we can only save part of the request body locally, there is no need to discard
all of it.

Yet, if checksums are enabled, we will still fail later with a 413.

src/tus_file.c

@@ -915,21 +915,21 @@ tus_suck_f(void *priv, unsigned flush, const void *ptr, ssize_t len)
 	assert(len > 0);
 
 	l = write(suck->fd, ptr, len);
+
+	if (l > 0) {
+		*suck->upload_offset += l;
+		if (*suck->upload_offset > suck->max) {
+			errno = EFBIG;
+			return (-1);
+		}
+	}
+
 	if (l != len)
-		l = -1;
+		return (-1);
 
 	if (suck->chksum)
 		tus_chksum_update(suck->ctx, suck->chksum, ptr, len);
 
-	if (l <= 0)
-		return (l);
-
-	*suck->upload_offset += l;
-	if (*suck->upload_offset > suck->max) {
-		errno = EFBIG;
-		return (-1);
-	}
-
 	return (0);
 }