RFR: 8284023: java.sun.awt.X11GraphicsDevice.getDoubleBufferVisuals() leaks XdbeScreenVisualInfo

[Zhengyu Gu]

Please review this small patch that fixes memory leak of `XdbeScreenVisualInfo` obtained from `XdbeGetVisualInfo()` method call.

Test:
- [x] jdk_awt

-------------

Commit messages:
 - 8284023: java.sun.awt.X11GraphicsDevice.getDoubleBufferVisuals() leaks XdbeScreenVisualInfo

Changes: https://git.openjdk.java.net/jdk/pull/8047/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8047&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284023
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8047.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8047/head:pull/8047

PR: https://git.openjdk.java.net/jdk/pull/8047