explicitly ignore the result of sv_utf8_downgrade()

Coverity complains about this because most other calls are
checked.

CIDs 353483, 353003, 353000, 352998, 352995

mathoms.c

@@ -344,7 +344,7 @@ Perl_sv_pvbyte(pTHX_ SV *sv)
 {
     PERL_ARGS_ASSERT_SV_PVBYTE;
 
-    sv_utf8_downgrade(sv, FALSE);
+    (void)sv_utf8_downgrade(sv, FALSE);
     return sv_pv(sv);
 }
 

mg.c

@@ -1264,7 +1264,7 @@ Perl_magic_setenv(pTHX_ SV *sv, MAGIC *mg)
         /* defined environment variables are byte strings; unfortunately
            there is no SvPVbyte_force_nomg(), so we must do this piecewise */
         (void)SvPV_force_nomg_nolen(sv);
-        sv_utf8_downgrade(sv, /* fail_ok */ TRUE);
+        (void)sv_utf8_downgrade(sv, /* fail_ok */ TRUE);
         if (SvUTF8(sv)) {
             Perl_ck_warner_d(aTHX_ packWARN(WARN_UTF8), "Wide character in %s", "setenv");
             SvUTF8_off(sv);

pp.c

@@ -3099,7 +3099,7 @@ PP(pp_oct)
          SV* const tsv = sv_2mortal(newSVsv(sv));
 
          SvUTF8_on(tsv);
-         sv_utf8_downgrade(tsv, FALSE);
+         (void)sv_utf8_downgrade(tsv, FALSE);
          tmps = SvPV_const(tsv, len);
     }
     if (PL_op->op_type == OP_HEX)
@@ -3689,7 +3689,7 @@ PP(pp_crypt)
           * Yes, we made this up.  */
          SV* const tsv = newSVpvn_flags(tmps, len, SVf_UTF8|SVs_TEMP);
 
-         sv_utf8_downgrade(tsv, FALSE);
+         (void)sv_utf8_downgrade(tsv, FALSE);
          tmps = SvPV_const(tsv, len);
     }
 #  ifdef USE_ITHREADS

sv.c

@@ -10299,7 +10299,7 @@ Perl_sv_pvbyten_force(pTHX_ SV *const sv, STRLEN *const lp)
     PERL_ARGS_ASSERT_SV_PVBYTEN_FORCE;
 
     sv_pvn_force(sv,lp);
-    sv_utf8_downgrade(sv,0);
+    (void)sv_utf8_downgrade(sv,0);
     *lp = SvCUR(sv);
     return SvPVX(sv);
 }