Silence static analyzer warning about null termination of a string

Silence static analyzer warning about null termination of a string

The buf[] array is initialized to zeros, so it was already null
terminated since the read() call did not allow the last character of the
buffer to be overwritten. Since that was apparently not enough to make
some static analyzers understand the design, use explicit null
termination after a successful read() call.

Signed-off-by: Jouni Malinen <j@w1.fi>