patch 8.0.0344: unlet command leaks memory

Problem:    Unlet command leaks memory. (Nikolai Pavlov)
Solution:   Free the memory on error. (closes vim#1497)

src/eval.c

@@ -2079,7 +2079,10 @@ get_lval(
 	    }
 	    /* existing variable, need to check if it can be changed */
 	    else if (var_check_ro(lp->ll_di->di_flags, name, FALSE))
+	    {
+		clear_tv(&var1);
 		return NULL;
+	    }
 
 	    if (len == -1)
 		clear_tv(&var1);

src/testdir/test_unlet.vim

@@ -17,3 +17,7 @@ func Test_not_existing()
   unlet! does_not_exist
   call assert_fails('unlet does_not_exist', 'E108:')
 endfunc
+
+func Test_unlet_fails()
+  call assert_fails('unlet v:["count"]', 'E46:')
+endfunc

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    344,
 /**/
     343,
 /**/