py/modstruct: Fix struct.unpack with unaligned offset of native type.

With this patch alignment is done relative to the start of the buffer that
is being unpacked, not the raw pointer value, as per CPython.

Fixes issue #3314.

py/modstruct.c

@@ -146,6 +146,7 @@ STATIC mp_obj_t struct_unpack_from(size_t n_args, const mp_obj_t *args) {
         }
         p += offset;
     }
+    byte *p_base = p;
 
     // Check that the input buffer is big enough to unpack all the values
     if (p + total_sz > end_p) {
@@ -164,7 +165,7 @@ STATIC mp_obj_t struct_unpack_from(size_t n_args, const mp_obj_t *args) {
             res->items[i++] = item;
         } else {
             while (cnt--) {
-                item = mp_binary_get_val(fmt_type, *fmt, &p);
+                item = mp_binary_get_val(fmt_type, *fmt, p_base, &p);
                 res->items[i++] = item;
             }
         }