eval: fix printing of NaN in eval fate test.

This fixes "make fate-eval" on MSVC builds. Without this, the test outputs
"-1.#NaN" instead of "nan" on MSVS 2010.

libavutil/eval.c

@@ -636,6 +636,9 @@ int main(int argc, char **argv)
         av_expr_parse_and_eval(&d, *expr,
                                const_names, const_values,
                                NULL, NULL, NULL, NULL, NULL, 0, NULL);
+        if (isnan(d))
+            printf("'%s' -> nan\n\n", *expr);
+        else
             printf("'%s' -> %f\n\n", *expr, d);
     }