BUG/MEDIUM: mux-h2: don't refrain from offering oneself a used buffer

Usually when calling offer_buffer(), we don't expect to offer it to
ourselves. But with h2 we have the same buffer_wait for the two directions
so we can unblock the recv path when completing a send(), or we can unblock
part of the mux buffer after sending the first few buffers that we managed
to collect. Thus it is important to always accept to wake up any requester.

A few parts of this patch could possibly be backported but earlier versions
already have other issues related to low-buffer condition so it's not sure
it's worth taking the risk to make things worse.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index c5a10df..e3c0bb0 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -428,7 +428,7 @@
 {
 	if (bptr->size) {
 		b_free(bptr);
-		offer_buffers(h2c->buf_wait.target, tasks_run_queue);
+		offer_buffers(NULL, tasks_run_queue);
 	}
 }
 
@@ -442,7 +442,7 @@
 		count++;
 	}
 	if (count)
-		offer_buffers(h2c->buf_wait.target, tasks_run_queue);
+		offer_buffers(NULL, tasks_run_queue);
 }
 
 /* returns the number of allocatable outgoing streams for the connection taking
@@ -2852,7 +2852,7 @@
 		}
 
 		if (released)
-			offer_buffers(h2c->buf_wait.target, tasks_run_queue);
+			offer_buffers(NULL, tasks_run_queue);
 
 		/* wrote at least one byte, the buffer is not full anymore */
 		h2c->flags &= ~(H2_CF_MUX_MFULL | H2_CF_DEM_MROOM);
@@ -3062,7 +3062,7 @@
 		}
 
 		if (released)
-			offer_buffers(h2c->buf_wait.target, tasks_run_queue);
+			offer_buffers(NULL, tasks_run_queue);
 	}
 
 	/* either we can release everything now or it will be done later once