This is a simple fix similar to [JDK-8237372](https://bugs.openjdk.java.net/browse/JDK-8237372). The NPE can occur if a MOUSE_DRAGGED is received by tab header without receiving MOUSE_PRESSED. The fix also corrects a typo error. `perfromDrag()` to `performDrag()`
------------- Commit messages: - 8241249: NPE in TabPaneSkin.perfromDrag Changes: https://git.openjdk.java.net/jfx/pull/160/files Webrev: https://webrevs.openjdk.java.net/jfx/160/webrev.00 Issue: https://bugs.openjdk.java.net/browse/JDK-8241249 Stats: 9 lines in 2 files changed: 7 ins; 0 del; 2 mod Patch: https://git.openjdk.java.net/jfx/pull/160.diff Fetch: git fetch https://git.openjdk.java.net/jfx pull/160/head:pull/160 PR: https://git.openjdk.java.net/jfx/pull/160
