Minor frame size fix

* If encoding in CBR don't care that the VBR_seek_table is empty when
calculating the frame size.

Change-Id: I8b1a8bce870fa4c9c20298887ebfc4b638142ad6
diff --git a/libmp3lame/VbrTag.c b/libmp3lame/VbrTag.c
index 3aaf8fd..ac67c22 100644
--- a/libmp3lame/VbrTag.c
+++ b/libmp3lame/VbrTag.c
@@ -895,8 +895,10 @@
     if (gfc->Class_ID != LAME_ID) {
         return 0;
     }
-    if (gfc->VBR_seek_table.pos <= 0) {
-        return 0;
+    if (gfp->VBR != vbr_off) {
+        if (gfc->VBR_seek_table.pos <= 0) {
+            return 0;
+        }
     }
     if (size < gfc->VBR_seek_table.TotalFrameSize) {
         return gfc->VBR_seek_table.TotalFrameSize;