Better argument validation

All pointer arguments are now tested for NULL. json_string() now also
tests that strdup() succeeds. This is to ensure that no NULL values
end up in data structures.

Also desribe the different sources of errors in documentation.

test/testprogs/test_array.c

@@ -27,6 +27,9 @@ int main()
     if(json_array_size(array) != 0)
         fail("empty array has nonzero size");
 
+    if(!json_array_append(array, NULL))
+        fail("able to append NULL");
+
     if(json_array_append(array, five))
         fail("unable to append");
 
@@ -54,6 +57,9 @@ int main()
     if(json_array_set(array, 0, seven))
         fail("unable to set value");
 
+    if(!json_array_set(array, 0, NULL))
+        fail("able to set NULL");
+
     if(json_array_size(array) != 2)
         fail("wrong array size");
 
@@ -85,16 +91,26 @@ int main()
             fail("got wrong value");
     }
 
-    json_array_set_new(array, 15, json_integer(123));
+    if(json_array_set_new(array, 15, json_integer(123)))
+        fail("unable to set new value");
+
     value = json_array_get(array, 15);
     if(!json_is_integer(value) || json_integer_value(value) != 123)
       fail("json_array_set_new works incorrectly");
 
-    json_array_append_new(array, json_integer(321));
+    if(!json_array_set_new(array, 15, NULL))
+        fail("able to set_new NULL value");
+
+    if(json_array_append_new(array, json_integer(321)))
+        fail("unable to append new value");
+
     value = json_array_get(array, json_array_size(array) - 1);
     if(!json_is_integer(value) || json_integer_value(value) != 321)
       fail("json_array_append_new works incorrectly");
 
+    if(!json_array_append_new(array, NULL))
+        fail("able to append_new NULL value");
+
     json_decref(five);
     json_decref(seven);
     json_decref(array);

test/testprogs/test_object.c

@@ -29,6 +29,12 @@ int main()
     if(json_object_set(object, "a", string))
         fail("unable to set value");
 
+    if(!json_object_set(object, NULL, string))
+        fail("able to set NULL key");
+
+    if(!json_object_set(object, "a", NULL))
+        fail("able to set NULL value");
+
     iter = json_object_iter(object);
     if(!iter)
         fail("unable to get iterator");
@@ -105,11 +111,19 @@ int main()
         fail("unable to set value");
 
 
-    json_object_set_new(object, "foo", json_integer(123));
+    if(json_object_set_new(object, "foo", json_integer(123)))
+        fail("unable to set new value");
+
     value = json_object_get(object, "foo");
     if(!json_is_integer(value) || json_integer_value(value) != 123)
       fail("json_object_set_new works incorrectly");
 
+    if(!json_object_set_new(object, NULL, json_integer(432)))
+        fail("able to set_new NULL key");
+
+    if(!json_object_set_new(object, "foo", NULL))
+        fail("able to set_new NULL value");
+
     json_decref(string);
     json_decref(other_string);
     json_decref(object);

test/testprogs/test_simple.c

@@ -58,6 +58,10 @@ int main()
         fail("invalid string value");
     json_decref(value);
 
+    value = json_string(NULL);
+    if(value)
+        fail("json_string(NULL) failed");
+
     value = json_integer(123);
     if(!value)
         fail("json_integer failed");