Parse nan in JSON as NaN instead of triggering a parse error (#2712)

* Fix memory leak for has(nan)

jv_array_get() used to be responsible of freeing the input array, but
since b5c4c3d67d, it is no longer called
if the key is nan. We need to free it manually to avoid leaking the
array.

* Parse  nan  in JSON as NaN instead of triggering a parse error

Fixes #2021

tests/jq.test

@@ -1799,6 +1799,18 @@ reduce .[] as $then (4 as $else | $else; . as $elif | . + $then * $elif)
 
 
 # { $__loc__ } works
+
 { a, $__loc__, c }
 {"a":[1,2,3],"b":"foo","c":{"hi":"hey"}}
 {"a":[1,2,3],"__loc__":{"file":"<top-level>","line":1},"c":{"hi":"hey"}}
+
+
+# nan is parsed as a valid NaN value from JSON
+
+fromjson | isnan
+"nan"
+true
+
+tojson | fromjson
+{"a":nan}
+{"a":null}