x86: Redundant test+jump in C1 allocateArray

In `C1_MacroAssembler::initialize_body()` we test the input array length for 0. We do this again in `MacroAssembler::zero_memory()`.

`MacroAssembler::zero_memory()` is only ever called from `C1_MacroAssembler::initialize_body()`.

We can eliminate one test+jump.