patch 9.1.0649: Wrong comment for "len" argument of call_simple_func()

Problem:  Wrong comment for "len" argument of call_simple_func().
Solution: Remove the "or -1 to use strlen()".  Also change its type to
          size_t to remove one cast. (zeertzjq)

closes: #15410

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/eval.c b/src/eval.c
index 4f56463..cc289b4 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -3259,7 +3259,7 @@
 	char_u *p = STRNCMP(arg, "<SNR>", 5) == 0 ? skipdigits(arg + 5) : arg;
 
 	if (to_name_end(p, TRUE) == parens)
-	    r = call_simple_func(arg, (int)(parens - arg), rettv);
+	    r = call_simple_func(arg, (size_t)(parens - arg), rettv);
     }
     return r;
 }