Protect get_exif_date_time_offset for non-compliant exiftool -OffsetTimeOriginal values,

[oPromessa]

For non-compliant exiftool -OffsetTimeOriginal values, protect get_exif_date_time_offset prior to callingexif_offset_to_seconds. Check if offset matches '+/-hh:mm' format.

Bumped into a JPG from a friend's camera with a value of " : " on OffsetTimeOriginal!!!

$ exiftool -a -s -G1 'IMG_20YYMMDD_144722.JPG' | egrep -i "off|model"
[MacOS]         MDItemAcquisitionModel          : E-M10 Mark III
[MacOS]         MDItemFlashOnOff                : 0
[MacOS]         MDItemLensModel                 : Olympus M.Zuiko Digital 45mm F1.8
[MacOS]         MDItemRedEyeOnOff               : 0
[IFD0]          Model                           : E-M10 Mark III
[ExifIFD]       OffsetTime                      :    :
[ExifIFD]       OffsetTimeOriginal              :    :
[ExifIFD]       OffsetTimeDigitized             :    :
[ExifIFD]       LensModel                       : OLYMPUS M.45mm F1.8

[RhetTbull]

Thanks! I have some other work I'm trying to get merged then will tackle your PRs.