Fix warning shift-negative-value

mrbgems/mruby-sprintf/src/sprintf.c

@@ -17,7 +17,7 @@
 
 #define BIT_DIGITS(N)   (((N)*146)/485 + 1)  /* log2(10) =~ 146/485 */
 #define BITSPERDIG MRB_INT_BIT
-#define EXTENDSIGN(n, l) (((~0 << (n)) >> (((n)*(l)) % BITSPERDIG)) & ~(~0 << (n)))
+#define EXTENDSIGN(n, l) (((~0U << (n)) >> (((n)*(l)) % BITSPERDIG)) & ~(~0U << (n)))
 
 mrb_value mrb_str_format(mrb_state *, int, const mrb_value *, mrb_value);
 static void fmt_setup(char*,size_t,int,int,mrb_int,mrb_int);