generator: Add validation check for return values that are arrays

When a C function returns an array, we need to be able to determine its
length. This is done either through a parameter to that function, or
because the array is null-terminated. If we don't know about either of
those, we print out a warning and fail the validation for the return
value. This means the corresponding method will not be generated.

This fixes a crash when trying to generate a method for which this
information is missing.

This wasn't a problem until now because the parser doesn't handle array
return values, so they were always handled through fix-ups. But now we
can get a GAPI XML converted from GIR which contains the "array"
attribute for a return value but no other information.

generator/ReturnValue.cs

@@ -182,6 +182,11 @@ namespace GtkSharp.Generation {
 			} else if ((CSType == "GLib.List" || CSType == "GLib.SList") && String.IsNullOrEmpty (ElementType))
 				log.Warn ("Returns {0} with unknown element type.  Add element_type attribute with gapi-fixup.", CType);
 
+			if (is_array && !is_null_term && String.IsNullOrEmpty (array_length_param)) {
+				log.Warn ("Returns an array with undeterminable length. Add null_term_array or array_length_param attribute with gapi-fixup.");
+				return false;
+			}
+
 			return true;
 		}
 	}