pp_sort: avoid potential I32 overflow from the comparator

Coverity says:

    CID 584092:         Integer handling issues  (INTEGER_OVERFLOW)
    Expression "result", where "Perl_SvIV(my_perl, *my_perl->Istack_sp)" is known to be equal to 0, overflows the type of "result", which is type "I32".

The sort comparison function returns IV (a Perl integer), but all the
sorting routines in this file want to work with I32. Instead of
converting (and possibly truncating) the value directly, normalize the
result to -1/0/1, which fits in any integer type.

Author: mauke

pp_sort.c

@@ -1167,7 +1167,6 @@ static I32
 S_sortcv(pTHX_ SV *const a, SV *const b)
 {
     const I32 oldsaveix = PL_savestack_ix;
-    I32 result;
     PMOP * const pm = PL_curpm;
     COP * const cop = PL_curcop;
     SV *olda, *oldb;
@@ -1191,7 +1190,11 @@ S_sortcv(pTHX_ SV *const a, SV *const b)
     /* entry zero of a stack is always PL_sv_undef, which
      * simplifies converting a '()' return into undef in scalar context */
     assert(PL_stack_sp > PL_stack_base || *PL_stack_base == &PL_sv_undef);
-    result = SvIV(*PL_stack_sp);
+    const IV iv = SvIV(*PL_stack_sp);
+    const I32 result =
+        iv > 0 ? 1 :
+        iv < 0 ? -1 :
+        0;
     rpp_popfree_to_NN(PL_stack_base);
 
     LEAVE_SCOPE(oldsaveix);
@@ -1206,7 +1209,6 @@ static I32
 S_sortcv_stacked(pTHX_ SV *const a, SV *const b)
 {
     const I32 oldsaveix = PL_savestack_ix;
-    I32 result;
     AV * const av = GvAV(PL_defgv);
     PMOP * const pm = PL_curpm;
     COP * const cop = PL_curcop;
@@ -1256,7 +1258,11 @@ S_sortcv_stacked(pTHX_ SV *const a, SV *const b)
     /* entry zero of a stack is always PL_sv_undef, which
      * simplifies converting a '()' return into undef in scalar context */
     assert(PL_stack_sp > PL_stack_base || *PL_stack_base == &PL_sv_undef);
-    result = SvIV(*PL_stack_sp);
+    const IV iv = SvIV(*PL_stack_sp);
+    const I32 result =
+        iv > 0 ? 1 :
+        iv < 0 ? -1 :
+        0;
     rpp_popfree_to_NN(PL_stack_base);
 
     LEAVE_SCOPE(oldsaveix);
@@ -1273,7 +1279,6 @@ S_sortcv_xsub(pTHX_ SV *const a, SV *const b)
 {
     const I32 oldsaveix = PL_savestack_ix;
     CV * const cv=MUTABLE_CV(PL_sortcop);
-    I32 result;
     PMOP * const pm = PL_curpm;
 
     PERL_ARGS_ASSERT_SORTCV_XSUB;
@@ -1291,7 +1296,11 @@ S_sortcv_xsub(pTHX_ SV *const a, SV *const b)
     /* entry zero of a stack is always PL_sv_undef, which
      * simplifies converting a '()' return into undef in scalar context */
     assert(PL_stack_sp > PL_stack_base || *PL_stack_base == &PL_sv_undef);
-    result = SvIV(*PL_stack_sp);
+    const IV iv = SvIV(*PL_stack_sp);
+    const I32 result =
+        iv > 0 ? 1 :
+        iv < 0 ? -1 :
+        0;
     rpp_popfree_to_NN(PL_stack_base);
 
     LEAVE_SCOPE(oldsaveix);