firmware_handler: prevent writing metadata w/o data handler

The notion of writing to metadata in this application is only relevant
if there's a data handler.  Add a check to verify there is a data
handler present, otherwise return false.

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: Ib2ffbf5c6560186e56e919a0ebc6ed1e2e8e04d3
diff --git a/firmware_handler.cpp b/firmware_handler.cpp
index c7164dc..8daead6 100644
--- a/firmware_handler.cpp
+++ b/firmware_handler.cpp
@@ -510,9 +510,14 @@
         return false;
     }
 
-    /* TODO: Prevent writing meta to the verification blob. */
+    /* Prevent writing meta to the verification blob (it has no data handler).
+     */
+    if (item->second->dataHandler)
+    {
+        return item->second->dataHandler->writeMeta(data);
+    }
 
-    return item->second->dataHandler->writeMeta(data);
+    return false;
 }
 
 /*