BUG/MAJOR: checks: segfault during tcpcheck_main

When using TCP health checks (tcp-check connect), it is possible to
crash with a segfault when, for reasons yet to be understood, the
protocol family is unknown.

In the function tcpcheck_main(), proto is dereferenced without a prior
test in case it is NULL, leading to the segfault during proto->connect
dereference.

The line has been unmodified since it was introduced, in commit
69e273f3fcfbfb9cc0fb5a09668faad66cfbd36b.  This was the only use of
proto (or more specifically, the return of  protocol_by_family()) that
was unprotected; all other callsites perform the test for a NULL
pointer.

This patch should be backported to 1.9, 1.8, 1.7, and 1.6.
1 file changed