integer-bytes->integer: repair for unsigned char

Fix `integer-bytes->integer` for the single-byte case when the C
compiler is configured to treat `char` as an unsigned type.

Relevant to #2773

racket/src/racket/src/numstr.c

@@ -2115,7 +2115,7 @@ Scheme_Object *scheme_bytes_to_integer(char *str, int slen, int sgned, int rshft
   switch(slen) {
   case 1:
     if (sgned) {
-      char val;
+      signed char val;
       memcpy(&val, str, sizeof(char));
       return scheme_make_integer(val);
     } else {