Fix infinite loop.

If a variable is a count of items, and we're looping while the count is
non-zero, we need to decrement it after every item we process.

Fixes Coverity CID 1435501.

Change-Id: Iabb0cb6276d4bcf4b1bdea9ec3ba943dac1b9938
Reviewed-on: https://code.wireshark.org/review/27402
Reviewed-by: Guy Harris <guy@alum.mit.edu>

epan/dissectors/packet-ieee80211.c

@@ -12212,6 +12212,7 @@ dissect_hs20_osen(tvbuff_t *tvb, packet_info *pinfo _U_, proto_tree *tree, void*
                           ENC_NA);
       offset += 16;
       pmkid_index++;
+      pmkid_count--;
     }
   }