media encoder: use resize to allocate buffer

Reserve doesn't allow to legally use the underlying buffer.

Change-Id: Ia76d4ce559c2efb6f1d9d471b2d68444258bed66

src/media/media_encoder.cpp

@@ -357,7 +357,7 @@ MediaEncoder::initStream(const SystemCodecInfo& systemCodecInfo, AVBufferRef* fr
         else if (scaledFrameBufferSize_ <= AV_INPUT_BUFFER_MIN_SIZE)
             throw MediaEncoderException("buffer too small");
 
-        scaledFrameBuffer_.reserve(scaledFrameBufferSize_);
+        scaledFrameBuffer_.resize(scaledFrameBufferSize_);
         scaledFrame_ = std::make_shared<VideoFrame>();
         scaledFrame_->setFromMemory(scaledFrameBuffer_.data(), format, width, height);
     }