pp_sleep: fix potential I32 overflow

Coverity says:

    CID 584095:         Integer handling issues  (INTEGER_OVERFLOW)
    Expression "duration", where "(IV)Perl_SvIV(my_perl, *sp--)" is known to be equal to 0, overflows the type of "duration", which is type "I32 const".

There are two dodgy type conversions in this function: from IV (POPi) to
I32 (duration), and from I32 (duration) to unsigned int (sleep argument,
by cast). Avoid the one Coverity complains about by making 'duration' an
IV.

Author: mauke

pp_sys.c

@@ -5123,7 +5123,7 @@ PP_wrapped(pp_sleep, MAXARG, 0)
     if (MAXARG < 1 || (!TOPs && !POPs))
         PerlProc_pause();
     else {
-        const I32 duration = POPi;
+        const IV duration = POPi;
         if (duration < 0) {
           /* diag_listed_as: %s() with negative argument */
           ck_warner_d(packWARN(WARN_MISC),