busybox

replace: count_strstr - Handle an edge case where sub is empty

If sub is empty, avoids an infinite loop.

function                                             old     new   delta
count_strstr                                          45      63     +18

Signed-off-by: Martin Lewis <martin.lewis.x84@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>