pp_substr: checking SvOK() is superfluous after SvPV_force_nomg()

When replacing some characters in pp_substr, there should be no
need to check `SvOK(sv)`, since previous calls to
`SvPV_force_nomg(sv)` and (if needed) `sv_utf8_upgrade_nomg(sv);`
should always return a `sv` that is `SvOK`.

pp.c

@@ -3438,8 +3438,11 @@ PP(pp_substr)
                 sv_utf8_upgrade(repl_sv_copy);
                 repl = SvPV_const(repl_sv_copy, repl_len);
             }
-            if (!SvOK(sv))
-                SvPVCLEAR(sv);
+
+            /* The earlier SvPV_force_nomg(sv, curlen) should have ensured
+             * that sv is SvOK, even if it wasn't beforehand. */
+            assert(SvOK(sv));
+
             sv_insert_flags(sv, byte_pos, byte_len, repl, repl_len, 0);
             SvREFCNT_dec(repl_sv_copy);
         }