jit: Fix accidentally-harmless type confusion

In 2a0faed, 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

Author: anarazel

src/backend/jit/llvm/llvmjit_expr.c

@@ -2060,7 +2060,7 @@ llvm_compile_expr(ExprState *state)
 					v_nullsp = l_ptr_const(nulls, l_ptr(TypeStorageBool));
 
 					/* create blocks for checking args */
-					b_checknulls = palloc(sizeof(LLVMBasicBlockRef *) * nargs);
+					b_checknulls = palloc(sizeof(LLVMBasicBlockRef) * nargs);
 					for (int argno = 0; argno < nargs; argno++)
 					{
 						b_checknulls[argno] =