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)

Author: brammool

src/blob.c

@@ -63,6 +63,7 @@ blob_copy(typval_T *from, typval_T *to)
     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)

src/testdir/test_blob.vim

@@ -71,6 +71,10 @@ func Test_blob_assign()
   call assert_fails('let b .= "xx"', 'E734:')
   call assert_fails('let b += "xx"', 'E734:')
   call assert_fails('let b[1:1] .= 0z55', 'E734:')
+
+  let l = [0z12]
+  let m = deepcopy(l)
+  let m[0] = 0z34	" E742 or E741 should not occur.
 endfunc
 
 func Test_blob_get_range()

src/version.c

@@ -777,6 +777,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1671,
 /**/
     1670,
 /**/