MEDIUM: sample: fill the struct sample with the session, proxy and stream pointers

Some sample analyzer (sample-fetch or converters) needs to known the proxy,
session and stream attached to the sampel. The sample-fetches and the converters
function pointers cannot be called without these 3 pointers filled.

This patch permits to reduce the sample-fetch and the converters called
prototypes, and provides a new mean to add information for this type of
functions.
diff --git a/src/sample.c b/src/sample.c
index 77192be..53c9d0d 100644
--- a/src/sample.c
+++ b/src/sample.c
@@ -1033,6 +1033,9 @@
 		memset(p, 0, sizeof(*p));
 	}
 
+	p->px   = px;
+	p->sess = sess;
+	p->strm = strm;
 	if (!expr->fetch->process(px, sess, strm, opt, expr->arg_p, p, expr->fetch->kw, expr->fetch->private))
 		return NULL;