Take more care to avoid leaving throw_svalue around with a backtrace.

Rev: src/builtin_functions.c:1.390
Rev: src/cpp.c:1.90
Rev: src/interpret.c:1.212
Rev: src/interpret.h:1.91
Rev: src/language.yacc:1.255
Rev: src/las.c:1.255
Rev: src/mapping.c:1.129
Rev: src/modules/Java/jvm.c:1.34
Rev: src/object.c:1.176
Rev: src/pike_error.h:1.12
Rev: src/preprocessor.h:1.43
Rev: src/svalue.c:1.110