libblkid: iso9660: allocate enough space for UTF16 decoding

libblkid: iso9660: allocate enough space for UTF16 decoding

When merge_utf16be_ascii() encounters high-codepoint surrogate pairs it
emits four bytes of output for one byte of ascii input.
In addition with the remaining ascii characters from the second loop we
need up to 5 * sizeof(input) / 2 bytes as output buffer.

As we decode up to 128 ascii characters with merge_utf16be_ascii() we
need 320 bytes of buffer available.

Furthermore adapt merge_utf16be_ascii() to not write paste the output
buffer end.

Fix OSS-Fuzz issue 53149 ( #1887 )