RAR5 reader: Fixed a read from invalid memory block

RAR5 reader: Fixed a read from invalid memory block

In multi-file RAR5 archives, if a block spans from one file to another,
the RAR5 reader merges both blocks into one, and feeds this merged block
to the decompressor function. The problem is that the block merge
function allocates the exact number of bytes for this block. This is
problematic because when trying to read the last byte from this new
block with bit reader functions, the bit reader functions will reference
few additional bytes right after the byte the caller is trying to read,
resulting in an out of bounds read.

The commit increases the allocation size for new merged block. This
ensures that bit reader functions will never perform any out of bounds
reads. Additional space is zeroed out to prevent errors from
instrumentation tools like ASan or Valgrind.

Fixes #1119