tag: exif: do not include \0 in size passed to g_convert

tag: exif: do not include \0 in size passed to g_convert

When using g_convert, we should only pass the length
of the string content (without the \0) as g_convert will
only parse the real contents when changing formats. Including
the \0 causes it to add another \0, increasing the string
size when not needed.

For example, when writting a North geo location ref entry, that should
be a string with a single N letter, it would write:
"N\0\0", causing the string to have size 3, instead of 2 as expected.

In our case, we can pass -1 and let g_convert calculate the strlen as
we don't use the length anywhere else.

This fixes jifmux's tests on gst-plugins-bad.