Fix memory leak with oneofs and PB_ENABLE_MALLOC (#615)

Nanopb would leak memory when all of the following conditions were true:
- PB_ENABLE_MALLOC is defined at the compile time
- Message definitions contains an oneof field,
  the oneof contains a static submessage, and
  the static submessage contains a pointer field.
- Data being decoded contains two values for the submessage.

The logic in pb_release_union_field would detect that the same
submessage occurs twice, and wouldn't release it because keeping
the old values is necessary to match the C++ library behavior
regarding message merges.

But then decode_static_field() would go to memset() the whole
submessage to zero, because it unconditionally assumed it to
be uninitialized memory. This would normally happen when the
contents of the union field is switched to a different oneof
item, instead of merging with the same one.

This commit changes it so that the field is memset() only when
`which_field` contains a different tag.

Author: PetteriAimonen

pb_decode.c

@@ -464,14 +464,17 @@ static bool checkreturn decode_static_field(pb_istream_t *stream, pb_wire_type_t
             }
 
         case PB_HTYPE_ONEOF:
-            *(pb_size_t*)iter->pSize = iter->pos->tag;
-            if (PB_LTYPE(type) == PB_LTYPE_SUBMESSAGE)
+            if (PB_LTYPE(type) == PB_LTYPE_SUBMESSAGE &&
+                *(pb_size_t*)iter->pSize != iter->pos->tag)
             {
                 /* We memset to zero so that any callbacks are set to NULL.
-                 * Then set any default values. */
+                 * This is because the callbacks might otherwise have values
+                 * from some other union field. */
                 memset(iter->pData, 0, iter->pos->data_size);
                 pb_message_set_to_defaults((const pb_field_t*)iter->pos->ptr, iter->pData);
             }
+            *(pb_size_t*)iter->pSize = iter->pos->tag;
+
             return func(stream, iter->pos, iter->pData);
 
         default: