avoid hGetMetered 0 closing the handle

This is an edge case, which happened to be triggered by the P2P protocol
seeing DATA 0. When reading 0 bytes, getting an empty string does
not mean the handle has reached EOF.

I verified there was in fact a bug, where get of an empty file followed
by another file would get the empty file and then fail
with "handle is closed". This fixes it.

This commit was sponsored by Boyd Stephen Smith Jr. on Patreon.

Utility/Metered.hs

@@ -169,8 +169,9 @@ hGetMetered h wantsize meterupdate = lazyRead zeroBytesProcessed
 		c <- S.hGet h (nextchunksize (fromBytesProcessed sofar))
 		if S.null c
 			then do
-				hClose h
-				return $ L.empty
+				when (wantsize /= Just 0) $
+					hClose h
+				return L.empty
 			else do
 				let !sofar' = addBytesProcessed sofar (S.length c)
 				meterupdate sofar'