gtimezone: Add sanity checks on tzdata file format
@pwithnall
Submitted by Philip Withnall Link to original bug (#756026)
Description
Patch attached to add a few g_return_if_fail()s after the offset calculations in the tzdata loading function, to bail if the tzdata file produces invalid offsets. It does not do further validation on the data (for example, it does not check any of the transitions). The idea is that this will prevent GLib roaming off into odd parts of memory if a corrupt tzdata file is loaded, and will cause it to abort gracefully instead.