Add missing error check in jsonpb's marshal implementation.

Otherwise invalid JSON can be produced.

jsonpb/jsonpb.go

diff --git a/jsonpb/jsonpb.go b/jsonpb/jsonpb.go
index 0ad60eb..40d96c4 100644
--- a/jsonpb/jsonpb.go
+++ b/jsonpb/jsonpb.go
@@ -382,7 +382,9 @@
 				out.write(m.Indent)
 				out.write(m.Indent)
 			}
-			m.marshalValue(out, prop, sliceVal, indent+m.Indent)
+			if err := m.marshalValue(out, prop, sliceVal, indent+m.Indent); err != nil {
+				return err
+			}
 			comma = ","
 		}
 		if m.Indent != "" {