BUG/MEDIUM: http: the "unique-id" sample fetch could crash without a steeam

BUG/MEDIUM: http: the "unique-id" sample fetch could crash without a steeam

Since commit 5f940703b3 ("MINOR: log: Don't depends on a stream to process
samples in log-format string") it has become quite obvious that a few sample
fetch functions and converters were still heavily dependent on the presence
of a stream without testing for it.

The unique-id sample fetch function, if called without a stream, will result
in a crash.

This fix adds a check for the stream's existence, and should be backported
to all stable versions up to 1.7.

(cherry picked from commit a1062a4de8ba300979cc380b69b284f86dc13853)
[wt: adjusted ctx: unique_id was a ptr not an ist in 2.1]
Signed-off-by: Willy Tarreau <w@1wt.eu>