MFC r258426: libexecinfo: Include terminating null in byte count

Otherwise, a formatted string with a strlen equal to the remaining
buffer space would have the last character omitted (because vsnprintf
always null-terminates), and later the assert in backtrace_symbols_fmt
would fail.

Sponsored by: DARPA, AFRL
Approved by: re (gjb)

MFC r258426: libexecinfo: Include terminating null in byte count

Otherwise, a formatted string with a strlen equal to the remaining
buffer space would have the last character omitted (because vsnprintf
always null-terminates), and later the assert in backtrace_symbols_fmt
would fail.

Sponsored by: DARPA, AFRL
Approved by: re (gjb)