* src/vm/builtin.c (builtin_throw_exception): Changed return type and

* src/vm/builtin.c (builtin_throw_exception): Changed return type and
return NULL. This is less error-prone in asm_vm_call_method.
* src/vm/builtin.h: Likewise.

* src/vm/jit/alpha/asmpart.S (asm_vm_call_method_exception_handler):
Don't set v0 to NULL. This is now done in builtin_throw_exception.
* src/vm/jit/i386/asmpart.S: Likewise.
* src/vm/jit/mips/asmpart.S: Likewise.
* src/vm/jit/powerpc/asmpart.S: Likewise.
* src/vm/jit/x86_64/asmpart.S: Likewise.