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

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>