unlokibootimg: Check that the ramdisk's gzip timestamp is not 0x00000000

Ktoonsez's 20140319 kernels have 0x1F8B0800 before the "real" gzip
header and it is followed by 0x00000000. In the gzip header, bytes
5 through 8 are for the timestamp. We'll work around this by
ensuring that the timestamp is not zero. GZip's specifications
allow having a zero timestamp, but I haven't seen any boot image
creation utility that does that.