libsnapshot: Fix crash in MakeXorBlockString test.

std::string doesn't like being initialized with negative values, so use
memset() instead.

Bug: 208944665
Test: vts_libsnapshot_test
Change-Id: I42a1a01fd9e59ffc9913c9df9f4bb8ab85113356
diff --git a/fs_mgr/libsnapshot/snapshot_reader_test.cpp b/fs_mgr/libsnapshot/snapshot_reader_test.cpp
index 078f16e..9adc655 100644
--- a/fs_mgr/libsnapshot/snapshot_reader_test.cpp
+++ b/fs_mgr/libsnapshot/snapshot_reader_test.cpp
@@ -155,8 +155,8 @@
     }
 
     std::string MakeXorBlockString() {
-        std::string data(100, -1);
-        data.resize(kBlockSize, 0);
+        std::string data(kBlockSize, 0);
+        memset(data.data(), 0xff, 100);
         return data;
     }