patch 8.2.0668: compiler warning for int/size_t usage

Problem:    Compiler warning for int/size_t usage.
Solution:   Change "int" to "size_t". (Mike Williams)
diff --git a/src/version.c b/src/version.c
index c57ef87..7382a3d 100644
--- a/src/version.c
+++ b/src/version.c
@@ -747,6 +747,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    668,
+/**/
     667,
 /**/
     666,
diff --git a/src/vim9execute.c b/src/vim9execute.c
index 5251d48..3cb8628 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -653,7 +653,7 @@
 	    case ISN_EXECCONCAT:
 		{
 		    int	    count = iptr->isn_arg.number;
-		    int	    len = 0;
+		    size_t  len = 0;
 		    int	    pass;
 		    int	    i;
 		    char_u  *cmd = NULL;