mruby-sprintf:fix double negative signs in printf; fix #3148

MRB_INT_MAX does not have corresponding positive integer

mrbgems/mruby-sprintf/src/sprintf.c

@@ -828,12 +828,8 @@ retry:
           }
         }
         if (sign) {
-          if (v < 0) {
-            v = -v;
-            sc = '-';
-            width--;
-          }
-          else if (flags & FPLUS) {
+          if (v > 0) {
+            if (flags & FPLUS) {
               sc = '+';
               width--;
             }
@@ -841,6 +837,7 @@ retry:
               sc = ' ';
               width--;
             }
+          }
           switch (base) {
           case 2:
             strncpy(nbuf, RSTRING_PTR(val), sizeof(nbuf));