t.init() should be called at the time of template creation
i.e, template.New() and t.New() instead of later in the process.

- Removed calls of t.init() from t.Parse(), t.Execute(), t.Funcs()
- Also got rid of t.common != nil checks as it should never be nil

Fixes #10879

Change-Id: I1b7ac812f02c841ae80037babce7e2b0a2df13e8
Reviewed-on: https://go-review.googlesource.com/10240Reviewed-by: Rob Pike <r@golang.org>