jit: Fix accidentally-harmless type confusion

 jit: Fix accidentally-harmless type confusion
 
 In 2a0faed9d702, which added JIT compilation support for expressions, I
 accidentally used sizeof(LLVMBasicBlockRef *) instead of
 sizeof(LLVMBasicBlockRef) as part of computing the size of an allocation. That
 turns out to have no real negative consequences due to LLVMBasicBlockRef being
 a pointer itself (and thus having the same size). It still is wrong and
 confusing, so fix it.
 
 Reported by coverity.
 
 Backpatch-through: 13