Pass 0 to __builtin_frame_address() to appease modern GCC.

Modern versions of GCC warn about passing non-zero values to
__builtin_frame_address(). Passing 1 is a cosmetic change to remove
the db_trace_self() frame from the printed stack trace.

Reported by: marius, imp
Differential Revision: https://reviews.freebsd.org/D22600