BUG/MEDIUM: cache/stats: Handle inbuf allocation failure in the I/O handler

BUG/MEDIUM: cache/stats: Handle inbuf allocation failure in the I/O handler

When cache and stats applets were changed to use their own buffers, a change
was also performed to no longer access the stream from the I/O
handller. Among other things, the HTTP start-line of the request is now
retrieved to get the method. But, when these changes were brought, the inbuf
buffer allocation failures were not handled.

It is of course not so common. But if this happens, a crash may be
experienced. To fix the issue, we now check for inbuf allocation failures
before accessing it.

No backported needed.