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fs: fix readb with missing block

If lookup returns Eexist, then there is no block for this part of
the file, and kv.v does not contain a block pointer.

Return early after zeroing the result in this case.

--- a/fs.c

+++ b/fs.c

@@ -254,6 +254,8 @@

 	fb = o & ~(Blksz-1);

 	fo = o & (Blksz-1);

+	if(fo+n > Blksz)

+		n = Blksz-fo;

 	k.k = buf;

 	k.nk = sizeof(buf);

@@ -262,21 +264,20 @@

 	PBIT64(k.k+9, fb);

 	e = lookup(f, &k, &kv, kvbuf, sizeof(kvbuf), 0);

-	if(e != nil && e != Eexist){

-		werrstr(e);

-		return -1;

+	if(e != nil){

+		if(e != Eexist){

+			werrstr(e);

+			return -1;

+		}

+		memset(d, 0, n);

+		return n;

 	}

 	bp = unpackbp(kv.v, kv.nv);

 	if((b = getblk(bp, GBraw)) == nil)

 		return -1;

-	if(fo+n > Blksz)

-		n = Blksz-fo;

-	if(b != nil){

-		memcpy(d, b->buf+fo, n);

-		putblk(b);

-	}else

-		memset(d, 0, n);

+	memcpy(d, b->buf+fo, n);

+	putblk(b);

 	return n;

 }