This is the same issue that 8a3c7197a2 was
trying to address, albeit incompletely.
The sps_t structure contains an array of 64 short_term_ref_pic_sets_t
elements. Therefore at most 63 may be added to the base pointer
short_term_ref_pic_sets for accessing one of those elements.
Fixes the following test cases of #1780:
explorer🆔000355,sig:11,src:000738,op:havoc,rep:4
explorer🆔000497,sig:11,src:001249,op:flip1,pos:101