virStorageSourceClear: Don't leave dangling pointers behind

Imagine that this function is called twice over the same disk
source. While in the first run all allocated memory is freed, not
all pointers are set to NULL (e.g. def->srcpool). So when called
again, these poitners are freed again resulting in double free.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

src/util/virstoragefile.c

@@ -2274,6 +2274,8 @@ virStorageSourceClear(virStorageSourcePtr def)
     VIR_FREE(def->nodeformat);
 
     virStorageSourceBackingStoreClear(def);
+
+    memset(def, 0, sizeof(*def));
 }