Fix Possible Null Pointer De-reference.

Inside macro :
#if defined( WBXML_ENCODER_USE_STRTBL ), encoder is dereferenced without Null check as below :
    strstbl_len = encoder->strstbl_len;

So, this may cause crash. Hence moved the Null check condition at appropriate place.
Fix Possible Null Pointer De-reference.

strstbl_len = encoder->strstbl_len;  can cause crash in de-referencing encoder, if Null check is done after this statement.
Applied Null check at appropriate place.
Merge pull request #1 from ya1gaurav/patch-1

Fix Possible Null Pointer De-reference.