RFR: 8331603: Cleanup native AbstractSurface methods getRGBImpl, setRGBImpl

[Ambarish Rapte]

The parameter "offset" is not validated in the 2 native methods getRGBImpl() and setRGBImpl() of com.sun.pisces.AbstractSurface (in JAbstractSurface.c).
The PR adds the "offset < 0" check to both the methods.

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Commit messages:
 - add missing check

Changes: https://git.openjdk.org/jfx/pull/1497/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1497&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331603
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/1497.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1497/head:pull/1497

PR: https://git.openjdk.org/jfx/pull/1497