Merge cherrypicks of ['googleplex-android-review.googlesource.com/23750798'] into tm-platform-release.

Change-Id: I0366d1d912ecc6f9afc919942787d05a85be0a0d
diff --git a/src/main/java/com/android/apksig/ApkVerifier.java b/src/main/java/com/android/apksig/ApkVerifier.java
index 8ae5f78..0b04ef9 100644
--- a/src/main/java/com/android/apksig/ApkVerifier.java
+++ b/src/main/java/com/android/apksig/ApkVerifier.java
@@ -1276,6 +1276,15 @@
         }
 
         private void mergeFrom(ApkSigningBlockUtils.Result source) {
+            if (source == null) {
+                return;
+            }
+            if (source.containsErrors()) {
+                mErrors.addAll(source.getErrors());
+            }
+            if (source.containsWarnings()) {
+                mWarnings.addAll(source.getWarnings());
+            }
             switch (source.signatureSchemeVersion) {
                 case ApkSigningBlockUtils.VERSION_APK_SIGNATURE_SCHEME_V2:
                     mVerifiedUsingV2Scheme = source.verified;