examples: wgetjson: check allocations before use

[Old-Ding]

Summary

• Check wgetjson response, JSON path, request buffer, and POST body allocations before use.
• Return or propagate allocation failures instead of dereferencing NULL buffers.

Root cause

Several malloc() results in the wgetjson example were used immediately by memcpy(), snprintf(), web_posts_str(), web_post_str(), or webclient_perform() without first checking whether allocation succeeded. Low-memory paths could therefore crash before the existing cleanup logic ran.

Testing

• git diff --check
• git show --stat --check --format=fuller HEAD

Not run: local compile, because this environment does not have make, cmake, gcc, clang, or cc available.

[xiaoxiang781216]

@Old-Ding please fix:

c1da0ee1f examples: wgetjson: check allocations before use
../nuttx/tools/checkpatch.sh -c -u -m -g 4d5f60005c6d91c76e4a585fe3102aab50119ec6..HEAD
❌ Missing git commit message
Used config files:
    1: .codespellrc
Some checks failed. For contributing guidelines, see:
  https://github.com/apache/nuttx/blob/master/CONTRIBUTING.md
Error: Process completed with exit code 1.

[cederom]

Thank you @Old-Ding, please git commit message and valid email as git commit signature.