As it can apparently happen with some camera software
(https://user-images.githubusercontent.com/2621/39392188-71a66fc4-4a66-11e8-9e3b-694163efa643.jpg),
sometimes a tag bytes will not only have a bunch of trailing NULLs, but
the Count for the data in that tag will be wrongly set too. i.e. instead
of being set to the actual number of relevant bytes to read, it will
be set to the full number of bytes, trailing zeroes included.
As it can apparently happen with some camera software (https://user-images.githubusercontent.com/2621/39392188-71a66fc4-4a66-11e8-9e3b-694163efa643.jpg), sometimes a tag bytes will not only have a bunch of trailing NULLs, but the Count for the data in that tag will be wrongly set too. i.e. instead of being set to the actual number of relevant bytes to read, it will be set to the full number of bytes, trailing zeroes included.
As a consequence, the Tag.strVal returned would include these trailing NULLs too, which would lead to bugs such as https://github.com/perkeep/perkeep/issues/1127
This change therefore makes sure to remove all trailing NULLs from the data, in the case of DTAscii at least.