Compiler.h: add gcc_returns_nonnull, gcc_returns_twice

src/pcm/PcmBuffer.hxx

@@ -47,11 +47,11 @@ public:
 	 * to signal "error".  An empty destination buffer is not
 	 * always an error.
 	 */
-	gcc_malloc
+	gcc_malloc gcc_returns_nonnull
 	void *Get(size_t size);
 
 	template<typename T>
-	gcc_malloc
+	gcc_malloc gcc_returns_nonnull
 	T *GetT(size_t n) {
 		return (T *)Get(n * sizeof(T));
 	}

src/pcm/SampleFormat.hxx

@@ -122,7 +122,7 @@ sample_format_size(SampleFormat format)
  * @param format a #SampleFormat enum value
  * @return the string
  */
-gcc_pure gcc_malloc
+gcc_pure gcc_malloc gcc_returns_nonnull
 const char *
 sample_format_to_string(SampleFormat format) noexcept;