Check for null fd when constructing AssetFileDescriptor.

An AssetFileDescriptor with a null fd is pretty useless and should
never happen during normal operation.  It can't even be parcelled
without hitting an NPE.  To make it easier to find bugs where a null
fd might have been generated, check at construction time.

Bug: 5655036
Change-Id: I1da6dbee0cab9c11e309f3cebe698bf44dc5e27d

Author: Jeff Brown

core/java/android/content/res/AssetFileDescriptor.java

@@ -52,6 +52,9 @@ public class AssetFileDescriptor implements Parcelable {
      */
     public AssetFileDescriptor(ParcelFileDescriptor fd, long startOffset,
             long length) {
+        if (fd == null) {
+            throw new IllegalArgumentException("fd must not be null");
+        }
         if (length < 0 && startOffset != 0) {
             throw new IllegalArgumentException(
                     "startOffset must be 0 when using UNKNOWN_LENGTH");