Set name_mapping to NULL after freeing

To avoid risk of double-free.
    
Fixes #237.
diff --git a/src/fcfreetype.c b/src/fcfreetype.c
index cd8fa43..bfbd5d7 100644
--- a/src/fcfreetype.c
+++ b/src/fcfreetype.c
@@ -1627,7 +1627,10 @@
 	}
     }
     if (!nm_share)
+    {
 	free (name_mapping);
+	name_mapping = NULL;
+    }
 
     if (!nfamily && face->family_name &&
 	FcStrCmpIgnoreBlanksAndCase ((FcChar8 *) face->family_name, (FcChar8 *) "") != 0)