RAR5 reader: fix a potential SIGSEGV on 32-bit builds

RAR5 reader: fix a potential SIGSEGV on 32-bit builds

The reader was causing a SIGSEGV when the file has been declaring a
specific dictionary size. Dictionary sizes above 0xFFFFFFFF bytes are
overflowing size_t type on 32-bit builds. In case the file has been
declaring dictionary size of 0x100000000 (so, UINT_MAX+1), the
window_size variable effectively contained value of 0. Later, the memory
allocation function was skipping actual allocation of 0 bytes, but still
tried to unpack the data.

This commit limits the dictionary window size buffer to 64MB, so it
always fits in a size_t variable, and disallows a zero dictionary size
for files in the header processing stage.

One unit test had to be modified after this change.