Fix uninitialized read of Texture::_format

Discovered by valgrind.
Call sequence leading to the problem:
Urho3D::FontFace::LoadFaceTexture calls CreateFaceTexture() and right afterwards SetData()
SetData will call IsCompressed() at line 316, thus reading potentially garbage data.

Source/Engine/Graphics/OpenGL/OGLTexture.cpp

@@ -72,6 +72,7 @@ Texture::Texture(Context* context) :
     Resource(context),
     GPUObject(GetSubsystem<Graphics>()),
     usage_(TEXTURE_STATIC),
+    format_(0),
     levels_(0),
     requestedLevels_(0),
     width_(0),