Fix dump-tzdata.sh for "zic -b slim" files

"zic -b slim" omits ttisgmt and ttisstd bytes. The old ("fat") format
always had them. This change is sufficient to enable dumping of slim
files.

Bug: 159583160
Test: Ran dump-tzdata.sh with a slim tzdata file and inspected the output
Change-Id: I234eebb41b4af0dfee70d9646eeb3a24fb753f7e
1 file changed