BUG/MEDIUM: h2: In h2_send(), stop the loop if we failed to alloc a buf.

In h2_send(), make sure we break the loop if we failed to alloc a buffer,
or we'd end up looping endlessly.

This should be backported to 1.9.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index afb3ad5..8f8c37b 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -2723,6 +2723,9 @@
 		while (((h2c->flags & (H2_CF_MUX_MFULL|H2_CF_MUX_MALLOC)) == 0) && !done)
 			done = h2_process_mux(h2c);
 
+		if (h2c->flags & H2_CF_MUX_MALLOC)
+			break;
+
 		if (conn->flags & CO_FL_ERROR)
 			break;