RFR: 6521141: DebugGraphics NPE @ setFont();

[Tejesh R]

`DebugGraphics` class has a Graphics instance which is been used in slowed down drawing. The `graphics` object is not initialized anywhere inside the class, where it is expected to set explicitly by the user. When the user doesn't set it and try to use the any mehtods like `drawing/setFont`, NPE is raised which is expected. The scenario is taken care by checking if the `graphics` object is null before using it inside the class, thus eliminating the NPE case.

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Commit messages:
 - Fix : graphics null check
 - Merge remote-tracking branch 'upstream/master'
 - Merge remote-tracking branch 'upstream/master'
 - Merge remote-tracking branch 'upstream/master'
 - Merge remote-tracking branch 'upstream/master'
 - Merge branch 'master' of github.com:TejeshR13/jdk
 - Merge branch 'openjdk:master' into master
 - Merge remote-tracking branch 'upstream/master'
 - Merge remote-tracking branch 'upstream/master'
 - Merge branch 'master' of github.com:TejeshR13/jdk
 - ... and 1 more: https://git.openjdk.org/jdk/compare/c7c20661...12511ec1

Changes: https://git.openjdk.org/jdk/pull/9673/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9673&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-6521141
  Stats: 279 lines in 1 file changed: 121 ins; 0 del; 158 mod
  Patch: https://git.openjdk.org/jdk/pull/9673.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9673/head:pull/9673

PR: https://git.openjdk.org/jdk/pull/9673