TIFF/EXIF tags: Fix potential 32bit unsigned integer overflow

[mdietsch]

...for rational type in metadata.

I ran into an issue with for example using 0.000493 secs for exposure time. This change fixes this.

[syoyo]

Thanks! Let me give some time to review the PR

[mdietsch]

This is the code-snippet I used to investigate this issue: godbolt
Line 11 lets you toggle between my fix and the original code

It may be of help

[syoyo]

This is the code-snippet I used to investigate this issue: godbolt Line 11 lets you toggle between my fix and the original code

It may be of help

Thanks! It saves my time to write a test! Confirmed the code, and it works. So merged!