[mongo_proxy]potential heap_buffer_overflow with untrusted buffer when onData()

#### Since the docs of mongo_proxy shows it is still an alpha version and doesn't allow untrusted input now, I think it is not a security issue, so I post it here.

An invalid input/untrusted input can cause heap_buffer_overflow if the buffer contains a "string" which doesn't end with `\0`.
```
std::string BufferHelper::removeString(Buffer::Instance& data) {
int32_t length = removeInt32(data);
if (static_cast(length) > data.length()) {
throw EnvoyException("invalid buffer size");
}

char* start = reinterpret_cast(data.linearize(length));
std::string ret(start);
data.drain(length);
return ret;
}
```
For example, in the failed test case, `length`=16 and `start` should be a string with length=16, however, if it doesn't end with `\0`, when executing `std::string ret(start)`, there will be `heap_buffer_overflow`.
A possible fix is to replace this line with `std::string ret(start, length)` which makes sure the length is explicitly specified.

(the code is here: https://github.com/envoyproxy/envoy/blob/master/source/extensions/filters/network/mongo_proxy/bson_impl.cc#L104)

The issue and test case can also be found here: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=24961&sort=-opened&can=1&q=proj%3Aenvoy%20status%3DNew