gtimer: Add overflow checks to g_time_val_from_iso8601()

gtimer: Add overflow checks to g_time_val_from_iso8601()

The code was previously doing a few bits of arithmetic without checking
whether they would overflow, and hence not validating the date/time it
was parsing very strictly.

The parser is now not 100% strict, but should avoid any arithmetic
overflows which would cause an invalid result to be returned even though
g_time_val_from_iso8601() had returned TRUE.

oss-fuzz#9673

Signed-off-by: Philip Withnall <withnall@endlessm.com>