platform: print_backtrace_to_buffer can overflow buffer

The platform function print_backtrace_to_buffer doesn't correctly check for buffer being full when printing backtraces. As a consequence it can overwrite the buffer (and corrupt the stack).

Note we only call print_backtrace_to_buffer as part of (fatal) error handling, so this isn't an expoit and and of itself, however it does mean we don't get very useful minidumps (as the stack has been trashed).

(Note: regression introduced by http://review.couchbase.org/55856, only present in watson).