BUG/MINOR: errors: invalid use of memprintf in startup_logs_init()

BUG/MINOR: errors: invalid use of memprintf in startup_logs_init()

On startup/reload, startup_logs_init() will try to export startup logs shm
filedescriptor through the internal HAPROXY_STARTUPLOGS_FD env variable.

While memprintf() is used to prepare the string to be exported via
setenv(), str_fd argument (first argument passed to memprintf()) could
be non NULL as a result of HAPROXY_STARTUPLOGS_FD env variable being
already set.

Indeed: str_fd is already used earlier in the function to store the result
of getenv("HAPROXY_STARTUPLOGS_FD").

The issue here is that memprintf() is designed to free the 'out' argument
if out != NULL, and here we don't expect str_fd to be freed since it was
provided by getenv() and would result in memory violation.

To prevent any invalid free, we must ensure that str_fd is set to NULL
prior to calling memprintf().

This must be backported in 2.7 with eba6a54cd4 ("MINOR: logs: startup-logs
can use a shm for logging the reload")