BUG/MEDIUM: config: avoid skipping disabled proxies

Paul Taylor and Bryan Talbot found that after commit 419ead8 ("MEDIUM:
config: compute the exact bind-process before listener's maxaccept"),
a backend marked "disabled" would cause the next backend to be skipped
and if it was the last one it would cause a segfault.

The reason is that the commit above changed the "while" loop for a "for"
loop but a "continue" statement still incrementing the current proxy was
left in the code for disabled proxies, causing the next one to be skipped
as well and the last one to try to dereference NULL when seeking ->next.

The quick workaround consists in not disabling backends, or adding an
empty dummy one after a disabled section.

This fix must be backported to 1.5.