Fix uninitialized memory access if a FShaderProgram is destroyed without being fully compiled and linked

src/gl/shaders/gl_shaderprogram.cpp

@@ -52,6 +52,12 @@
 #include "i_system.h"
 #include "doomerrors.h"
 
+FShaderProgram::FShaderProgram()
+{
+	for (int i = 0; i < NumShaderTypes; i++)
+		mShaders[i] = 0;
+}
+
 //==========================================================================
 //
 // Free shader program resources

src/gl/shaders/gl_shaderprogram.h

@@ -6,6 +6,7 @@
 class FShaderProgram
 {
 public:
+	FShaderProgram();
 	~FShaderProgram();
 
 	enum ShaderType
@@ -30,6 +31,9 @@ public:
 	static void PatchFragShader(FString &code);
 
 private:
+	FShaderProgram(const FShaderProgram &) = delete;
+	FShaderProgram &operator=(const FShaderProgram &) = delete;
+
 	static FString PatchShader(ShaderType type, const FString &code, const char *defines, int maxGlslVersion);
 	static void PatchCommon(FString &code);