pp_hot.c: Don't use signed char as an array index

The MingW compiler spotted this.  The code prior to these uses already
has made sure these indices are positive, so casting to unsigned works.

Author: khwilliamson

pp_hot.c

@@ -359,7 +359,7 @@ PP(pp_aelemfastlex_store)
 
     /* inlined av_fetch() for simple cases ... */
     if (!SvRMAGICAL(av) && key >=0 && key <= AvFILLp(av)) {
-        targ = AvARRAY(av)[key];
+        targ = AvARRAY(av)[ (U8) key ];
     }
     /* ... else do it the hard way */
     if (!targ) {
@@ -2033,7 +2033,7 @@ PP(pp_aelemfast)
 
     /* inlined av_fetch() for simple cases ... */
     if (!SvRMAGICAL(av) && key >= 0 && key <= AvFILLp(av)) {
-        sv = AvARRAY(av)[key];
+        sv = AvARRAY(av)[ (U8) key ];
         if (sv)
             goto ret;
         if (!lval) {