Server: Fix two issues identified by ASan

1. If the TLSPlain and X509Plain security types were both disabled, then
   rfbOptPamAuth() would overflow the name field in the secTypes
   structure when testing the "none" security type, since the name of
   that security type has less than five characters.  This issue was
   innocuous, since the overflow was fully contained within the secTypes
   structure, but the ASan error caused Xvnc to abort, which made it
   difficult to detect other errors.

2. If an ill-behaved RFB client sent the TurboVNC Server a fence
   message with more than 64 bytes, then the TurboVNC Server would
   try to read that message and subsequently overflow the stack before
   it detected that the payload was too large.  This could never have
   occurred with any of the VNC viewers that currently support the RFB
   flow control extensions (TigerVNC and TurboVNC, namely.)  This issue
   was also innocuous, since the stack overflow affected two variables
   (newScreens and errMsg) that were never accessed before the function
   returned.