patch 8.0.1003: 64 bit compiler warning

Problem:    64 bit compiler warning
Solution:   Add type cast. (Mike Williams)
diff --git a/src/channel.c b/src/channel.c
index 886522b..508fb0c 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -4890,7 +4890,7 @@
     int		has_spaces = FALSE;
 
     /* First count the number of extra bytes required. */
-    slen = STRLEN(arg);
+    slen = (int)STRLEN(arg);
     dlen = slen;
     for (s = arg; *s != NUL; MB_PTR_ADV(s))
     {