eval: test isnan(sqrt(-1)) instead of just sqrt(-1)

sqrt(-1) returns "some NaN", it's not specified which exactly.

libavutil/eval.c

@@ -608,7 +608,7 @@ int main(int argc, char **argv)
         "ceil(123.123)",
         "ceil(-123.123)",
         "sqrt(1764)",
-        "sqrt(-1)",
+        "isnan(sqrt(-1))",
         "not(1)",
         "not(NAN)",
         "not(0)",

tests/ref/fate/eval

@@ -136,8 +136,8 @@ Evaluating 'ceil(-123.123)'
 Evaluating 'sqrt(1764)'
 'sqrt(1764)' -> 42.000000
 
-Evaluating 'sqrt(-1)'
-'sqrt(-1)' -> -nan
+Evaluating 'isnan(sqrt(-1))'
+'isnan(sqrt(-1))' -> 1.000000
 
 Evaluating 'not(1)'
 'not(1)' -> 0.000000