CLEANUP: l7-retries: do not test the buffer before calling b_alloc()

The return value is enough now to know if the allocation succeeded or
failed.

This cleanup was already pushed by Willy (f499f50) but a revert crushed
it. It may be backported to the 2.4 because the original patch was done on
this version.

(cherry picked from commit 0fe1864f7db556f99f3a715a8a383902ed36bfae)
Signed-off-by: Willy Tarreau <w@1wt.eu>
diff --git a/src/stream_interface.c b/src/stream_interface.c
index cd01840..4400123 100644
--- a/src/stream_interface.c
+++ b/src/stream_interface.c
@@ -740,9 +740,7 @@
 			if (!s->txn || (s->txn->req.msg_state != HTTP_MSG_DONE))
 				si->flags &= ~SI_FL_L7_RETRY;
 			else {
-				if (b_is_null(&si->l7_buffer))
-					b_alloc(&si->l7_buffer);
-				if (b_is_null(&si->l7_buffer))
+				if (b_alloc(&si->l7_buffer) == NULL)
 					si->flags &= ~SI_FL_L7_RETRY;
 				else {
 					memcpy(b_orig(&si->l7_buffer),