Do not discard the whole tombstone on a "Caused by:" line (#892)

Keys are matched as bare prefixes, so `Caused by:` inside an abort message
reaches the `Cause` key, fails the key comparison and raises — and the
per-line loop turns that into an error that drops the entire text
tombstone, stack trace included.

A key mismatch means "this line is not that key", not "this file is
broken": decline the line and let the remaining keys have their turn. A
line with no colon is declined the same way instead of raising on the
unpack. The same trap has a second form in the field, HiSilicon/Huawei
tombstones printing `code around pc:` against the `code` key.

tests/android/test_artifact_tombstone_caused_by.py

@@ -0,0 +1,54 @@
+# Mobile Verification Toolkit (MVT)
+# Copyright (c) 2021-2023 The MVT Authors.
+# Use of this software is governed by the MVT License 1.1 that can be found at
+#   https://license.mvt.re/1.1/
+"""A `Caused by:` line must not discard the whole text tombstone.
+
+Keys are matched as bare prefixes, so `Caused by: …` — an ordinary line inside
+an abort message — reached the `Cause` key, failed the key comparison and
+raised, which `Tombstones.run()` logged while dropping the entire crash record.
+Seen on a 1.6 MB tombstone whose protobuf twin was zero bytes: the crash then
+had no representation at all.
+"""
+
+import datetime
+
+from mvt.android.artifacts.tombstone_crashes import TombstoneCrashArtifact
+
+TOMBSTONE = b"""\
+*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
+Build fingerprint: 'Xiaomi/vili_eea/vili:13/TKQ1.220829.002/V14.0.10.0:user/release-keys'
+Revision: '0'
+ABI: 'arm64'
+Timestamp: 2023-08-24 14:54:47.999124034+0300
+Process uptime: 12199s
+Cmdline: com.example.game
+pid: 8044, tid: 26222, name: UnityMain  >>> com.example.game <<<
+uid: 10235
+signal 6 (SIGABRT), code -1 (SI_QUEUE), fault addr --------
+Abort message: 'No pending exception expected: java.lang.SecurityException: listen
+  at void android.os.Parcel.readException() (Parcel.java:2920)
+Caused by: android.os.RemoteException: Remote stack trace:
+\tat com.android.server.TelephonyRegistry.listen(TelephonyRegistry.java:1096)
+"""
+
+WITH_CAUSE = TOMBSTONE + b"Cause: null pointer dereference\n"
+
+
+class TestTombstoneCausedBy:
+    def _parse(self, content):
+        artifact = TombstoneCrashArtifact()
+        artifact.results = []
+        artifact.parse("tombstone_23", datetime.datetime(2023, 8, 24), content)
+        return artifact.results
+
+    def test_caused_by_line_does_not_discard_the_tombstone(self):
+        results = self._parse(TOMBSTONE)
+        assert len(results) == 1
+        assert results[0]["pid"] == 8044
+        assert results[0]["process_name"] == "UnityMain"
+        assert results[0]["uid"] == 10235
+
+    def test_the_real_cause_key_is_still_parsed(self):
+        results = self._parse(WITH_CAUSE)
+        assert results[0]["cause"] == "null pointer dereference"