Macros: remove pointer math on a null pointer

Causes warnings on some compilers and/or tools.
Changed ACPI_TO_POINTER to use ACPI_CAST_PTR instead of using
arithmetic.

Reported by: Qian Cai (cai@lca.pw).

source/include/actypes.h

@@ -657,7 +657,7 @@ typedef UINT64                          ACPI_INTEGER;
 
 /* Pointer/Integer type conversions */
 
-#define ACPI_TO_POINTER(i)              ACPI_ADD_PTR (void, (void *) 0, (ACPI_SIZE) (i))
+#define ACPI_TO_POINTER(i)              ACPI_CAST_PTR (void, (ACPI_SIZE) (i))
 #define ACPI_TO_INTEGER(p)              ACPI_PTR_DIFF (p, (void *) 0)
 #define ACPI_OFFSET(d, f)               ACPI_PTR_DIFF (&(((d *) 0)->f), (void *) 0)
 #define ACPI_PHYSADDR_TO_PTR(i)         ACPI_TO_POINTER(i)