Return nil immediately if somebody tries to parse nil. (Otherwise, Ba…

…d Things will happen when we try to parse the C string.)

Reported by @cruffenach: boredzo/iso-8601-date-formatter#7

ISO8601DateFormatter.m

@@ -155,6 +155,9 @@ - (NSDateComponents *) dateComponentsFromString:(NSString *)string timeZone:(out
 	return [self dateComponentsFromString:string timeZone:outTimeZone range:NULL fractionOfSecond:NULL];
 }
 - (NSDateComponents *) dateComponentsFromString:(NSString *)string timeZone:(out NSTimeZone **)outTimeZone range:(out NSRange *)outRange fractionOfSecond:(out NSTimeInterval *)outFractionOfSecond {
+	if (string == nil)
+		return nil;
+
 	NSDate *now = [NSDate date];
 
 	NSDateComponents *components = [[[NSDateComponents alloc] init] autorelease];