fix Cursor::skipAtMostSlow bug introduced in D1724111

Test Plan: fbconfig -r folly/io/test && fbmake runtests_opt -j32

Reviewed By: viswanath@fb.com

Subscribers: folly-diffs@

FB internal diff: D1728927

Tasks: 5788015

Signature: t1:1728927:1418161580:9ed66412e35c62486a56e19aa9bf119863b5f1a1

Blame Revision: D1724111

folly/io/Cursor.h

@@ -441,7 +441,7 @@ class CursorBase {
       if (UNLIKELY(!tryAdvanceBuffer())) {
         return skipped;
       }
-      len -= skipped;
+      len -= available;
     }
     offset_ += len;
     return skipped + len;