fix: undefined behaviour in allocator

[xydone]

I would like to preface this by saying I have limited experience with high-performance C, so there is a possibility this is intended UB.

I was working on porting yyjson to the Zig build system when I ran into the following error by ubsan when running the test cases:

panic: applying non-zero offset 32 to null pointer
dyn_ctx *ctx = (dyn_ctx *)(void *)(alc + 1);

I have remedied the issue by moving the if(unlikely(!alc)) check before the pointer arithmetic.

Tested on the test suite from the docs, respecting -Wdeclaration-after-statement.