Fix return value of visibilitymap_get_status().

This function is declared as returning a uint8, but it returns a
bool in one code path.  To fix, return (uint8) 0 instead of false
there.  This should behave exactly the same as before, but it might
prevent future compiler complaints.

Oversight in commit a892234f83.

Author: Julien Rouhaud <rjuju123@gmail.com>
Discussion: https://postgr.es/m/aIHluT2isN58jqHV%40jrouhaud

src/backend/access/heap/visibilitymap.c

@@ -364,7 +364,7 @@ visibilitymap_get_status(Relation rel, BlockNumber heapBlk, Buffer *vmbuf)
 	{
 		*vmbuf = vm_readbuf(rel, mapBlock, false);
 		if (!BufferIsValid(*vmbuf))
-			return false;
+			return (uint8) 0;
 	}
 
 	map = PageGetContents(BufferGetPage(*vmbuf));