Reset sh.arithrecursion in sh_exit() instead (re: d50d3d7c)

Since the arithmetic recursion level only becomes incorrect when
an error interrupts the arithmetic subsystem, and all such error
messages call sh_exit(), it should be good enough to reset it
there, so we don't need to do that for nearly every sh_exec() run.

src/cmd/ksh93/sh/fault.c

@@ -580,6 +580,7 @@ void sh_exit(register int xno)
 	sfclrlock(sfstdin);
 	if(!pp)
 		sh_done(shp,sig);
+	shp->arithrecursion = 0;
 	shp->prefix = 0;
 #if SHOPT_TYPEDEF
 	shp->mktype = 0;