Set u.ext_data to null after it has been freed

SEGV_MAPPER crash could happen in MetaData::typed_data::freeStorage.
The faulty address could be both random pointers and deadbaad. It
seems like free could be called more than once on u.ext_data. Set
u.ext_data to null after it has been freed as there already is a
null check before trying to free it.

Change-Id: I710c49f9c231bd8852159914c0b2add58a685f0e
diff --git a/media/libstagefright/MetaData.cpp b/media/libstagefright/MetaData.cpp
index a01ec97..ae6ae2d 100644
--- a/media/libstagefright/MetaData.cpp
+++ b/media/libstagefright/MetaData.cpp
@@ -282,6 +282,7 @@
     if (!usesReservoir()) {
         if (u.ext_data) {
             free(u.ext_data);
+            u.ext_data = NULL;
         }
     }