Revert an incorrect substitution optimization introduced in 5.10.0. (Closes: #501178)

[perl #52658]

Bug introduced by upstream change 26334, reverted with change 33685 in blead
and 33732 in maint-5.10.
diff --git a/pp_ctl.c b/pp_ctl.c
index 7a377f0..88269a7 100644
--- a/pp_ctl.c
+++ b/pp_ctl.c
@@ -218,7 +218,6 @@ PP(pp_substcont)
 	if (!(cx->sb_rxtainted & 2) && SvTAINTED(TOPs))
 	    cx->sb_rxtainted |= 2;
 	sv_catsv(dstr, POPs);
-	FREETMPS; /* Prevent excess tmp stack */
 
 	/* Are we done */
 	if (cx->sb_once || !CALLREGEXEC(rx, s, cx->sb_strend, orig,