Fix potential memory corruption in NuCachedSource2.

Change-Id: Ia6bd2d4966287228a3ac1b067ee1f1ccad965dbb
related-to-bug: 4354845
diff --git a/media/libstagefright/NuCachedSource2.cpp b/media/libstagefright/NuCachedSource2.cpp
index dc86885..81f2e47 100644
--- a/media/libstagefright/NuCachedSource2.cpp
+++ b/media/libstagefright/NuCachedSource2.cpp
@@ -450,6 +450,11 @@
         }
 
         size_t avail = mCache->totalSize() - delta;
+
+        if (avail > size) {
+            avail = size;
+        }
+
         mCache->copy(delta, data, avail);
 
         return avail;