RFR: 8365609: Fix several potential NULL native pointer dereferences in the desktop module [v3]

[Artem Semenov]

> The defect has been detected and confirmed in the function OGLBlitToSurfaceViaTexture() located in the file src/java.desktop/share/native/common/java2d/opengl/OGLBlitLoops.c with static code analysis. This defect can potentially lead to a null pointer dereference.
> 
> The pointer pf is dereferenced in line 324 without checking for nullptr, although earlier in line 274 the same pointer is checked for nullptr, which indicates that it can be null.
> 
> In the same file, line 551 calls OGLBlitToSurfaceViaTexture() from line 263, where NULL is passed in place of pf.
> All other calls are fine.
> 
> Also, another function with a similar issue from the same file, OGLBlitSwToTexture() from line 396.
> 
> In src/java.desktop/unix/native/libawt_xawt/awt/gtk3_interface.c gtk3_load()
> The pointer fp_glib_check_version can be null, but it is dereferenced without any check. Although in the same file, for example, line 280 contains a check, this check does not lead to termination of execution.
> 
> 
> In src/java.desktop/share/native/libsplashscreen/splashscreen_gif.c SplashDecodeGif()
> The pointer colorMap is dereferenced after it has been checked against nullptr in lines 151 and 206. Moreover, between these checks and the mentioned location (line 282), the pointer is not modified in any way.
> 
> According to [this](https://github.com/openjdk/jdk/pull/26002#issuecomment-3023050372) comment, this PR contains fixes for similar cases in other places.

Artem Semenov has updated the pull request incrementally with one additional commit since the last revision:

  Update src/java.desktop/share/native/libsplashscreen/splashscreen_gif.c
  
  Co-authored-by: Aleksandr Zvegintsev <77687766+azvegint at users.noreply.github.com>

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/26799/files
  - new: https://git.openjdk.org/jdk/pull/26799/files/a7307652..f0a276f4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26799&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26799&range=01-02

  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/26799.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26799/head:pull/26799

PR: https://git.openjdk.org/jdk/pull/26799