GAPI: fix uninitialized variables in Fluid

modules/gapi/include/opencv2/gapi/fluid/gfluidbuffer.hpp

@@ -87,7 +87,7 @@ public:
 
 private:
     std::unique_ptr<Priv> m_priv;
-    const Cache* m_cache;
+    const Cache* m_cache = nullptr;
 };
 
 class GAPI_EXPORTS Buffer

modules/gapi/src/backends/fluid/gfluidbuffer_priv.hpp

@@ -245,7 +245,7 @@ class GAPI_EXPORTS Buffer::Priv
 
     // Coordinate starting from which this buffer is assumed
     // to be read (with border not being taken into account)
-    int m_readStart;
+    int m_readStart = 0;
     cv::Rect m_roi;
 
     friend void debugBufferPriv(const Buffer& p, std::ostream &os);