Bug 4748: r15240 broke IP-based URLs, twice. (#41)

Bug 4748: r15240 broke IP-based URLs, twice. (#41)

r15240 broke ipcacheCheckNumeric() because that function's static
storage was no longer reset properly between calls.

This bug is very different from bug 4741, but their symptoms (e.g.,
false "Host header forgery" SECURITY ALERTs) can be the same.

I did not realize that std::vector::resize(n, x) ignores x when the
vector size is already at least n. It is not a reset()-like method. My
tests did not have enough different IP-based URLs to expose this bug.