Fix buffer mutation in C++ access modifiers.

Author: Janfel

c++-mode/private

@@ -1,5 +1,6 @@
 # -*- mode: snippet -*-
 # name: private
+# expand-env: ((yas-also-auto-indent-first-line t))
 # --
-private:`(progn (indent-according-to-mode) "")`
+private:
     $0

c++-mode/protected

@@ -1,5 +1,6 @@
 # -*- mode: snippet -*-
 # name: protected
+# expand-env: ((yas-also-auto-indent-first-line t))
 # --
-protected:`(progn (indent-according-to-mode) "")`
+protected:
     $0

c++-mode/public

@@ -1,5 +1,6 @@
 # -*- mode: snippet -*-
 # name: public
+# expand-env: ((yas-also-auto-indent-first-line t))
 # --
-public:`(progn (indent-according-to-mode) "")`
+public:
     $0