HHVM-183: toJSON() should throw if bson_as_json() fails

This passes tests for PHPC-613.

Author: jmikola

src/MongoDB/BSON/functions.cpp

@@ -99,6 +99,11 @@ Variant HHVM_FUNCTION(MongoDBBsonToJson, const String &data)
 
 		str = bson_as_json(b, &str_len);
 
+		if (!str) {
+			bson_reader_destroy(reader);
+			throw MongoDriver::Utils::throwUnexpectedValueException("Could not convert BSON document to a JSON string");
+		}
+
 		s = String(str_len, ReserveString);
 		data_s = (unsigned char*) s.bufferSlice().data();
 		memcpy(data_s, str, str_len);