Reset sh.arithrecursion in sh_exit() instead (re: d50d3d7)

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;

src/cmd/ksh93/sh/xec.c

@@ -978,7 +978,6 @@ int sh_exec(register const Shnode_t *t, int flags)
 			error_info.line = t->com.comline-shp->st.firstline;
 			com = sh_argbuild(shp,&argn,&(t->com),OPTIMIZE);
 			echeck = 1;
-			shp->arithrecursion = 0;
 			if(t->tre.tretyp&COMSCAN)
 			{
 				argp = t->com.comarg;
@@ -1890,7 +1889,6 @@ int sh_exec(register const Shnode_t *t, int flags)
 
 		    case TPAR:
 			echeck = 1;
-			shp->arithrecursion = 0;
 			flags &= ~OPTIMIZE_FLAG;
 			if(!shp->subshell && !shp->st.trapcom[0] && !shp->st.trap[SH_ERRTRAP] && (flags&sh_state(SH_NOFORK)))
 			{
@@ -1960,7 +1958,6 @@ int sh_exec(register const Shnode_t *t, int flags)
 			int	*exitval=0,*saveexitval = job.exitval;
 			pid_t	savepgid = job.curpgid;
 			echeck = 1;
-			shp->arithrecursion = 0;
 			job.exitval = 0;
 			job.curjobid = 0;
 			if(shp->subshell)
@@ -2638,7 +2635,6 @@ int sh_exec(register const Shnode_t *t, int flags)
 				skipexitset++;
 			error_info.line = t->tst.tstline-shp->st.firstline;
 			echeck = 1;
-			shp->arithrecursion = 0;
 			if((type&TPAREN)==TPAREN)
 			{
 				sh_exec(t->lst.lstlef,OPTIMIZE);