Fix allocation formula in llvmjit_expr.c

An array of LLVMBasicBlockRef is allocated with the size used for an
element being "LLVMBasicBlockRef *" rather than "LLVMBasicBlockRef".
LLVMBasicBlockRef is a type that refers to a pointer, so this did not
directly cause a problem because both should have the same size, still
it is incorrect.

This issue has been spotted while reviewing a different patch, and
exists since 2a0faed9d7, so backpatch all the way down.

Discussion: https://postgr.es/m/CA+hUKGLngd9cKHtTUuUdEo2eWEgUcZ_EQRbP55MigV2t_zTReg@mail.gmail.com
Backpatch-through: 14

src/backend/jit/llvm/llvmjit_expr.c

@@ -612,8 +612,8 @@ llvm_compile_expr(ExprState *state)
 						LLVMBuildStore(b, l_sbool_const(1), v_resnullp);
 
 						/* create blocks for checking args, one for each */
-						b_checkargnulls =
-							palloc(sizeof(LLVMBasicBlockRef *) * op->d.func.nargs);
+						b_checkargnulls = (LLVMBasicBlockRef *)
+							palloc(sizeof(LLVMBasicBlockRef) * op->d.func.nargs);
 						for (int argno = 0; argno < op->d.func.nargs; argno++)
 							b_checkargnulls[argno] =
 								l_bb_before_v(b_nonull, "b.%d.isnull.%d", opno,