fixup! Check return value from snprintf(); ref #3498

Author: matz

Diff for: mrbgems/mruby-sprintf/src/sprintf.c

@@ -1049,7 +1049,10 @@ mrb_str_format(mrb_state *mrb, int argc, const mrb_value *argv, mrb_value fmt)
             need = width;
 
           CHECK(need + 1);
-          snprintf(&buf[blen], need + 1, "%*s", need, "");
+          n = snprintf(&buf[blen], need + 1, "%*s", need, "");
+          if (n < 0) {
+            mrb_raise(mrb, E_RUNTIME_ERROR, "formatting error");
+          }
           if (flags & FMINUS) {
             if (!isnan(fval) && fval < 0.0)
               buf[blen++] = '-';