tidy-up: drop stray casts for allocated pointers

[vszakats]

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Initial version of this patch tried to add missing NULL checks
to three allocations touched (in example and UV debug code).
I backtracked, because later the NULL pointers were derefed
anyway.

[Copilot]

Pull request overview

This PR tidies up C allocations across curl’s codebase and documentation by removing unnecessary casts from malloc/calloc/realloc-family calls, and it starts adding allocation-failure handling in a few example/debug-style code paths.

Changes:

• Remove stray pointer casts on allocation/reallocation calls across lib/, src/, tests/, projects/, and docs/.
• Add initial allocation-failure guards in libuv/libevent example and tool debug code paths (but some call sites still need NULL handling).
• Minor formatting/line-wrapping adjustments around realloc calls.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/libtest/lib2302.c	Removes an unnecessary cast from a curlx_calloc() assignment in a websocket libtest.
src/tool_operate.c	Refactors libuv context allocation; adds a NULL guard inside the allocator (call site still needs to handle NULL).
src/tool_formparse.c	Removes an unnecessary cast from a curlx_calloc() allocation.
src/tool_cb_wrt.c	Removes an unnecessary cast from a curlx_realloc() call in the Windows console path.
src/mkhelp.pl	Removes an unnecessary cast in generated zlib allocator glue code.
projects/OS400/curlmain.c	Removes an unnecessary cast from malloc() in OS/400 argument conversion main.
projects/OS400/curlcl.c	Removes an unnecessary cast from malloc() in the OS/400 CL wrapper (already checks allocation).
projects/OS400/ccsidcurl.c	Removes unnecessary casts from malloc()/calloc() in OS/400 CCSID getinfo handling.
lib/vtls/schannel.c	Removes unnecessary casts from curlx_malloc() allocations.
lib/vtls/schannel_verify.c	Removes unnecessary casts from curlx_malloc() allocations.
lib/vquic/cf-ngtcp2.c	Removes an unnecessary cast and rewraps a curlx_realloc() call.
lib/mime.c	Removes unnecessary casts and slightly refactors declarations around curlx_malloc().
lib/content_encoding.c	Removes an unnecessary cast in the zlib zalloc callback implementation.
docs/FAQ.md	Removes an unnecessary cast in an example realloc() call (reveals an unsafe realloc pattern needing adjustment).
docs/examples/multi-uv.c	Refactors allocation + adds NULL guard in libuv example context creation (caller still needs to handle NULL).
docs/examples/multi-event.c	Refactors allocation + adds NULL guard in libevent example context creation (caller still needs to handle NULL).

Comments suppressed due to low confidence (1)

docs/FAQ.md:1024

• This FAQ example assigns realloc() directly to mem->memory. If realloc() fails, the original pointer is lost (leak) and subsequent cleanup cannot free it. Use a temporary pointer and only update mem->memory after success; on failure, return 0 to signal a write error.

  mem->memory = realloc(mem->memory, mem->size + realsize + 1);
  if(mem->memory) {
    memcpy(&(mem->memory[mem->size]), ptr, realsize);
    mem->size += realsize;
    mem->memory[mem->size] = 0;

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[vszakats]

True, but it's debug/example code and I don't feel going the extra mile here.
Also how to fail in this case? Leaving this for a future fix if someone is interested.

[vszakats]

Or perhaps this patch should not attempt to touch this at all, since it makes
these OOM/NULL-deref issues less visible.

[vszakats]

Rolling back NULL checks.

[vszakats]

True, but not attempting to fix this in untestable code.