Fix OOB Read when writing TIFF with custom Metadata

[wiredfool]

(triggered in Tests/test_file_libtiff.py::TestFileLibTiff::test_custom_metadata)

Valgrind output:

**88** **********************************************************************
**88** Tests/test_file_libtiff.py::TestFileLibTiff::test_custom_metadata
**88** **********************************************************************
==88== Invalid read of size 1
==88==    at 0x890596C: _TIFFSetupFields (in /usr/lib/x86_64-linux-gnu/libtiff.so.5.5.0)
==88==    by 0x8904EEC: TIFFDefaultDirectory (in /usr/lib/x86_64-linux-gnu/libtiff.so.5.5.0)
==88==    by 0x890504C: TIFFCreateDirectory (in /usr/lib/x86_64-linux-gnu/libtiff.so.5.5.0)
==88==    by 0x8912141: ??? (in /usr/lib/x86_64-linux-gnu/libtiff.so.5.5.0)
==88==    by 0x89126EF: TIFFRewriteDirectory (in /usr/lib/x86_64-linux-gnu/libtiff.so.5.5.0)
==88==    by 0x891A2ED: TIFFFlush (in /usr/lib/x86_64-linux-gnu/libtiff.so.5.5.0)
==88==    by 0x87BD5CC: ImagingLibTiffEncode (TiffDecode.c:678)
==88==    by 0x877D519: _encode (encode.c:141)

Passing in a stack allocated array is going to fail, as a reference
is retained to the name and used later when flushing the Tiff to
the file.

[radarhere]

Thanks