The malloc size calculation in get_or_create_temp_file() allocates
strlen(tmp_dir) + strlen(prefix) + strlen(data.digest) + 2 bytes,
but the constructed path "tmp_dir/prefix-digest\0" requires +3 bytes
(for '/', '-', and the NUL terminator), not +2.

This causes a 1-byte heap-buffer-overflow when strcat writes the NUL
terminator of data.digest past the end of the allocated buffer.

Make the arithmetic explicit by writing each component's length
separately (+1 for each separator and the terminator), matching the
four strcat calls that follow.

The sister function create_temp_file() is not affected: its format
"tmp_dir/prefix.XXXXXX" only has one separator ('/'), and ".XXXXXX"
is already measured by strlen(".XXXXXX").

Fixes #492