Limit lenght to INT_MAX bytes in rfbProcessFileTransferReadBuffer()

This ammends 15bb719c03cc70f14c36a843dcb16ed69b405707 fix for a heap
out-of-bound write access in rfbProcessFileTransferReadBuffer() when
reading a transfered file content in a server. The former fix did not
work on platforms with a 32-bit int type (expected by rfbReadExact()).

CVE-2018-15127
<https://github.com/LibVNC/libvncserver/issues/243>
<https://github.com/LibVNC/libvncserver/issues/273>