Replace AssertMacro() with Assert() when not in macro

This was forgotten to be changed in commit 9c727360bcc.

src/include/storage/bufmgr.h

@@ -384,7 +384,7 @@ BufferGetBlock(Buffer buffer)
 static inline Size
 BufferGetPageSize(Buffer buffer)
 {
-	AssertMacro(BufferIsValid(buffer));
+	Assert(BufferIsValid(buffer));
 	return (Size) BLCKSZ;
 }