Add null check before for loop using the list

The bug was introduced by https://chromium-review.googlesource.com/c/chromium/src/+/2032427
and code was moved to here by https://chromium-review.googlesource.com/c/chromium/src/+/2063037

Bug: 1053790
Change-Id: I683351902f635ba329c14ccaec87540c36758cdf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2075418Reviewed-by: Gayane Petrosyan <gayane@chromium.org>
Commit-Queue: Gang Wu <gangwu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745076}

chrome/browser/share/android/java/src/org/chromium/chrome/browser/share/ShareImageFileUtils.java

@@ -64,10 +64,13 @@ public class ShareImageFileUtils {
 
         boolean anyChildKept = false;
         if (file.isDirectory()) {
+            File[] file_list = file.listFiles();
+            if (file_list != null) {
                 for (File child : file.listFiles()) {
                     anyChildKept |= deleteFiles(child, reservedFilepath);
                 }
             }
+        }
 
         // file.delete() will fail if |file| is a directory and has a file need to keep. In this
         // case, the log should not been recorded since it is correct.