RFR: 8241249: NPE in TabPaneSkin.perfromDrag

[Ambarish Rapte]

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()`

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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