Fix errors after enabling -Wpointer-arith
diff --git a/py/objarray.c b/py/objarray.c
index 2d210d9..614cbe1 100644
--- a/py/objarray.c
+++ b/py/objarray.c
@@ -27,6 +27,7 @@
 
 #include <string.h>
 #include <assert.h>
+#include <stdint.h>
 
 #include "mpconfig.h"
 #include "nlr.h"
@@ -292,7 +293,7 @@
             #endif
             } else {
                 res = array_new(o->typecode, slice.stop - slice.start);
-                memcpy(res->items, o->items + slice.start * sz, (slice.stop - slice.start) * sz);
+                memcpy(res->items, (uint8_t*)o->items + slice.start * sz, (slice.stop - slice.start) * sz);
             }
             return res;
 #endif
@@ -331,7 +332,7 @@
             // read-only memoryview
             return 1;
         }
-        bufinfo->buf += (mp_uint_t)o->free * sz;
+        bufinfo->buf = (uint8_t*)bufinfo->buf + (mp_uint_t)o->free * sz;
     }
     #endif
     return 0;