Resolve clang analyzer warning for localtime.c

[jengelh]

If malloc() yielded NULL but also left or set errno to 0 (under
very un-POSIX-y circumstances like private malloc implementations),
then tzload would effectively signal success to its caller though
there it was no success, and code branches would execute that were
not meant to be executed.

clang said:

localtime.c:321:16: warning: The right operand of '<' is a garbage
value [core.UndefinedBinaryOperatorResult]
for (i = 0; i < sp->charcnt; ++i)

[eggert]

I replied on the tz mailing list.