[NaCl SDK] nacl_io: Fix gaping memory leaks in memfs and htmlfs

I introduced these leaks recently and really should
have run valgrind.  In future out valgrind bot should
find such things pre-submit.

Review URL: https://codereview.chromium.org/605433003

Cr-Commit-Position: refs/heads/master@{#296606}

native_client_sdk/src/libraries/nacl_io/httpfs/http_fs_node.cc

@@ -236,6 +236,10 @@ HttpFsNode::HttpFsNode(Filesystem* filesystem,
   SetMode(S_IRALL);
 }
 
+HttpFsNode::~HttpFsNode() {
+  free(buffer_);
+}
+
 Error HttpFsNode::GetStat_Locked(struct stat* stat) {
   // Assume we need to 'HEAD' if we do not know the size, otherwise, assume
   // that the information is constant.  We can add a timeout if needed.

native_client_sdk/src/libraries/nacl_io/httpfs/http_fs_node.h

@@ -43,6 +43,8 @@ class HttpFsNode : public Node {
              const std::string& url,
              bool cache_content);
 
+  virtual ~HttpFsNode();
+
  private:
   Error GetStat_Locked(struct stat* stat);
   Error OpenUrl(const char* method,

native_client_sdk/src/libraries/nacl_io/memfs/mem_fs_node.cc

@@ -37,6 +37,7 @@ MemFsNode::MemFsNode(Filesystem* filesystem)
 }
 
 MemFsNode::~MemFsNode() {
+  free(data_);
 }
 
 Error MemFsNode::Read(const HandleAttr& attr,