Add missing tm_isdst initialization

[olebole]

Forwarded from @aurel32 Debian#939523:

As discussed in Debian#939048, the autopkgtest from iraf-fitsutil fail in a strange way when run with glibc 2.29 instead of glibc 2.28:

cl> fitsutil
This is the initial release of the IRAF FITSUTIL package
to include support for FITS tile compression via 'fpack'.
Please send comments and questions to [email protected].

cl> copy dev$pix.pix pix.pix
cl> copy dev$pix.imh pix.imh
cl> fgwrite "pix.pix pix.imh" pix.fits verb-
cl> mkdir out
cl> cd out
cl> fgread ../pix.fits "" "" verb-
cl> sum32 *
ERROR: No write permission on file (String_File)
   "directory (img, long+) | scan (junk, junk, filsiz)"
      line 42: fitsutil$src/sum32.cl
      called as: `sum32 (input=*)'
      called as: `cl ()'
   "clbye()"
      line 41: fitsutil$fitsutil.cl
      called as: `fitsutil ()'
      called as: `cl ()'
 Error while reading login.cl file - may need to rebuild with mkiraf
 Fatal startup error.  CL dies.

This happens because the error checking in mktime() have been improved in case a non-valid date is provided in the tm struct. More precisely in fgread.c, it should be noted that strptime does NOT setup the tm_isdst of tm struct, which is instead getting a random value from the stack. For this field 0 means no DST, positive value means DST and negative values means that the value should be computed by mktime().

In the iraf-fitsutil, tm_isdst is not known from the file so it should be set to -1, just like it's done in the POSIX.1-2018 strptime example:

https://pubs.opengroup.org/onlinepubs/9699919799/

Therefore the following patch fixes the issue.

Thank you @aurel32 for the quick fix!