Handle zero snapshot size appropriately.

This won't happen in practice because device size is never
zero. Hypothetically, if it is, PartitionCowCreator still
suggests that a CoW size of 8192 bytes needs to be created. In
reality, it is not necessary.

Test: libsnapshot_fuzzer
Bug: 155484992
Change-Id: I6c69f54820522d50c699384eec90c474ca3a9402
(cherry picked from commit bc84824e21acfdc83590d6d0d4827bd9854fc0f5)
Merged-In: I6c69f54820522d50c699384eec90c474ca3a9402
diff --git a/fs_mgr/libsnapshot/partition_cow_creator.cpp b/fs_mgr/libsnapshot/partition_cow_creator.cpp
index efdb59f..0df5664 100644
--- a/fs_mgr/libsnapshot/partition_cow_creator.cpp
+++ b/fs_mgr/libsnapshot/partition_cow_creator.cpp
@@ -181,6 +181,13 @@
     ret.snapshot_status.set_device_size(target_partition->size());
     ret.snapshot_status.set_snapshot_size(target_partition->size());
 
+    if (ret.snapshot_status.snapshot_size() == 0) {
+        LOG(INFO) << "Not creating snapshot for partition " << ret.snapshot_status.name();
+        ret.snapshot_status.set_cow_partition_size(0);
+        ret.snapshot_status.set_cow_file_size(0);
+        return ret;
+    }
+
     // Being the COW partition virtual, its size doesn't affect the storage
     // memory that will be occupied by the target.
     // The actual storage space is affected by the COW file, whose size depends