reading jbig2_decode_symbol_dict() source code:

A) note the if condition at line ..: 
      /* 6.5.5 (4c.ii) */
      if (!params->SDHUFF || params->SDREFAGG)
      {
         ..
      }
   We obviously enter the if body also in the case: 
      params->SDHUFF == params->SDREFAGG == TRUE          (1)

   In this case the variable Jbig2ArithState *as is still set to NULL because
   it is allocated only if params->SDHUFF == FALSE

B) in the body of if statement we found a call to routine       
   jbig2_decode_text_region() 
   Because of A) point, in the case (1) above-mentioned it receives a NULL 
   value for the parameter as.

C) jbig2_decode_text_region() calls in turn routine        
   jbig2_decode_refinement_region(). 
   In the case (1) also this call is done with a NULL value for the as
   parameter but jbig2_decode_refinement_region() uses this parameter and we 
   have a NULL-pointer dereference error.

Suming up: in the case params->SDHUFF == params->SDREFAGG == TRUE we can have a
GPFault in jbig2_decode_symbol_dict().

I don'know if the case (1) is possible in real situations however, if A)-C)
points are true, it seems to me a dangerous path of execution.