Fix AUPT: Native crash in /system/bin/mediaserver

Protect against invalid memcpy to NULL

bug: 15163357
Change-Id: Ifab17714473f45cb714a0ee765638733b3236ec8

proprietary/wvm/WVMMediaSource.cpp

@@ -477,6 +477,11 @@ void WVMMediaSource::DecryptCallback(WVEsSelector esType, void* input, void* out
     DecryptContext &context = source->getDecryptContext();
     uint32_t copied = length;
 
+    if (!context.mMediaBuf->data()) {
+        ALOGE("WVMMediaSource::DecryptCallback - no media buffer!");
+        return;
+    }
+
     if (clientContext->getCryptoPluginMode()) {
         // just determine crypto unit boundaries
         if (key) {