patch 8.1.1671: copying a blob may result in it being locked

Problem:    Copying a blob may result in it being locked.
Solution:   Reset v_lock. (Ken Takata, closes #4648)
diff --git a/src/blob.c b/src/blob.c
index cbd26b9..fb40a57 100644
--- a/src/blob.c
+++ b/src/blob.c
@@ -63,6 +63,7 @@
     int	    ret = OK;
 
     to->v_type = VAR_BLOB;
+    to->v_lock = 0;
     if (from->vval.v_blob == NULL)
 	to->vval.v_blob = NULL;
     else if (rettv_blob_alloc(to) == FAIL)