Reserve some stack room to give handle_error a fighting chance to report

out-of-stack errors without getting them itself.

Rev: src/backend.c:1.60
Rev: src/interpret.c:1.173
Rev: src/interpret.h:1.68
Rev: src/las.c:1.222
Rev: src/main.c:1.107
Rev: src/program.c:1.282
Rev: src/threads.c:1.145