Handle potential missing DTSTART property

(cherry picked from commit 362f3f9)

src/main/java/net/fortuna/ical4j/model/component/Observance.java

@@ -155,12 +155,21 @@ public final Date getLatestOnset(final Date date) {
 
         if (initialOnset == null) {
             try {
-                initialOnset = applyOffsetFrom(calculateOnset(((DtStart) getProperty(Property.DTSTART)).getDate()));
+                DtStart dtStart = (DtStart) getProperty(Property.DTSTART);
+                if (dtStart == null) {
+                    throw new ConstraintViolationException("Missing DTSTART property");
+                }
+                initialOnset = applyOffsetFrom(calculateOnset(dtStart.getDate()));
             } catch (ParseException e) {
                 Logger log = LoggerFactory.getLogger(Observance.class);
                 log.error("Unexpected error calculating initial onset", e);
                 // XXX: is this correct?
                 return null;
+            } catch (ConstraintViolationException e) {
+                Log log = LogFactory.getLog(Observance.class);
+                log.error("Unexpected error calculating initial onset", e);
+                // XXX: is this correct?
+                return null;
             }
         }
 

src/test/groovy/net/fortuna/ical4j/data/CalendarBuilderSpec.groovy

@@ -0,0 +1,67 @@
+package net.fortuna.ical4j.data
+
+import net.fortuna.ical4j.model.Calendar
+import net.fortuna.ical4j.util.CompatibilityHints
+import spock.lang.Specification
+
+import java.nio.charset.Charset
+
+/**
+ * Created by fortuna on 4/07/2016.
+ */
+class CalendarBuilderSpec extends Specification {
+
+    def 'test relaxed parsing'() {
+        given: 'a calendar object string'
+        String cal2 = '''BEGIN:VCALENDAR
+CALSCALE:GREGORIAN
+PRODID:-//xxxx//xxxxxx//EN
+VERSION:2.0
+BEGIN:VTIMEZONE
+TZID:(GMT-03:00)
+BEGIN:STANDARD
+DTSTART:16010101T230000
+TZOFFSETFROM:-0200
+TZOFFSETTO:-0300
+RRULE:FREQ=YEARLY;BYMONTH=2;BYDAY=3SU
+END:STANDARD
+BEGIN:DAYLIGHT
+TZOFFSETFROM:-0300
+TZOFFSETTO:-0200
+RRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=4SU
+END:DAYLIGHT
+END:VTIMEZONE
+BEGIN:VEVENT
+UID:1047216278.801463535396580.JavaMail.root(a)xxxx.xxxxx.xxxx
+LAST-MODIFIED:20160519T090547Z
+DTSTAMP:20160519T090547Z
+DTSTART;TZID="(GMT-03:00)":20160518T123000
+DTEND;TZID="(GMT-03:00)":20160518T133000
+X-MICROSOFT-CDO-ALLDAYEVENT:FALSE
+X-MICROSOFT-CDO-IMPORTANCE:1
+PRIORITY:5
+TRANSP:OPAQUE
+X-MICROSOFT-CDO-BUSYSTATUS:BUSY
+SEQUENCE:0
+SUMMARY:xxxxxxx S-xxxxx
+X-SCALIX-LABEL:0
+CLASS:PUBLIC
+END:VEVENT
+END:VCALENDAR
+'''
+
+        and: 'relaxed parsing is enabled'
+        CompatibilityHints.setHintEnabled(CompatibilityHints.KEY_RELAXED_UNFOLDING, true)
+
+        when: 'the calendar string is parsed'
+        System.out.println(cal2);
+        InputStream stream = new ByteArrayInputStream(cal2.getBytes(Charset.forName("UTF-8")));
+        CalendarBuilder builder = new CalendarBuilder();
+        Calendar calendar = null;
+
+        calendar = builder.build(stream);
+
+        then: 'the result is as expected'
+        calendar as String == cal2.replaceAll('\n', '\r\n')
+    }
+}