Fix/allocator size overflow

[Sn0wyDay]

Problem

The allocator aligns each requested size and adds an internal block header. For values near SIZE_MAX, these calculations can wrap and make an oversized request appear to be a small allocation.

As a result, js_malloc_rt() can return an undersized block for an unrepresentable request. The same issue affects allocation and reallocation paths, while the default allocator's memory-limit checks also contain arithmetic that can wrap before comparison.

Fix

• Reject sizes that cannot accommodate alignment and the largest internal block header.
• Apply the validation to allocation and reallocation paths.
• Rewrite the default allocator's limit checks using checked subtraction.
• Add regression coverage for SIZE_MAX allocation, zeroed allocation, and reallocation requests.

A failed reallocation continues to preserve the original allocation and its contents.

Testing

• make -j8 test
• make CONFIG_ASAN=y CONFIG_UBSAN=y -j8 test
• LTO and warning-as-error builds