[Rd] Patch for R to fix some buffer overruns and add a missing PROTECT().

[Karl Millar]

This patch is against current svn and contains three classes of fix:
   - Ensure the result is properly terminated after calls to strncpy()
   - Replace calls of sprintf() with snprintf()
   - Added a PROTECT() call in do_while which could cause memory
errors if evaluating the condition results in a warning.

Thanks,

Karl