Fix a CCE in MutablePublicArray

Discovered while validating 2.4.0 release (#1639)

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=313488001

Author: cushon

core/src/main/java/com/google/errorprone/bugpatterns/MutablePublicArray.java

@@ -28,6 +28,7 @@
 import com.google.errorprone.bugpatterns.BugChecker.VariableTreeMatcher;
 import com.google.errorprone.matchers.Description;
 import com.google.errorprone.matchers.Matcher;
+import com.sun.source.tree.ExpressionTree;
 import com.sun.source.tree.Tree.Kind;
 import com.sun.source.tree.VariableTree;
 import com.sun.tools.javac.tree.JCTree.JCLiteral;
@@ -66,10 +67,11 @@ private static boolean nonEmptyArrayMatcher(VariableTree arrayExpression, Visito
     if (!isArrayType().matches(arrayExpression, state)) {
       return false;
     }
-    JCNewArray newArray = (JCNewArray) arrayExpression.getInitializer();
-    if (newArray == null) {
+    ExpressionTree initializer = arrayExpression.getInitializer();
+    if (!(initializer instanceof JCNewArray)) {
       return false;
     }
+    JCNewArray newArray = (JCNewArray) initializer;
     if (!newArray.getDimensions().isEmpty()) {
       return newArray.getDimensions().stream()
           .allMatch(

core/src/test/java/com/google/errorprone/bugpatterns/MutablePublicArrayTest.java

@@ -168,4 +168,17 @@ public void publicStaticFinalStaticInitializeBlock() {
             "}")
         .doTest();
   }
+
+  @Test
+  public void notAnArray() {
+    compilationHelper
+        .addSourceLines(
+            "Test.java",
+            "class Test {",
+            "  // BUG: Diagnostic contains: MutablePublicArray",
+            "  public static final long[] y = {0};",
+            "  public static final long[] l = y;",
+            "}")
+        .doTest();
+  }
 }