fs: fix infinite loop in fs.readFileSync()

Fix an infinite loop in the case where the file got truncated by a concurrent
writer while fs.readFileSync() was busy reading in the file.

lib/fs.js

@@ -221,12 +221,7 @@ fs.readFileSync = function(path, encoding) {
     }
 
     pos += bytesRead;
-
-    if (size !== 0) {
-      done = pos >= size;
-    } else {
-      done = bytesRead >= 0;
-    }
+    done = (bytesRead === 0) || (size !== 0 && pos >= size);
   }
 
   fs.closeSync(fd);
@@ -234,6 +229,8 @@ fs.readFileSync = function(path, encoding) {
   if (size === 0) {
     // data was collected into the buffers list.
     buffer = Buffer.concat(buffers, pos);
+  } else if (pos < size) {
+    buffer = buffer.slice(0, pos);
   }
 
   if (encoding) buffer = buffer.toString(encoding);