RFR: 6521141: DebugGraphics NPE @ setFont(); [v6]

[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.

Tejesh R has updated the pull request incrementally with two additional commits since the last revision:

 - Updations on removing StackWalker for graphics instance and test updations based on review
 - Minor <p> update

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9673/files
  - new: https://git.openjdk.org/jdk/pull/9673/files/22bc13a4..d9d62c86

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9673&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9673&range=04-05

  Stats: 31 lines in 2 files changed: 1 ins; 26 del; 4 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