Reduce dependencies on global variables.

Got lost in the previous commit.

This compatibility hack was accidently broken by commit f072cbbb

memcmp() can overflow the buffer.

Regression by commit 738583e3

The state ERROR can only be entered during decoder initialization, and
Seek() may only be called after initialization has been finished.

Some optimized implementations of memcmp() may not start from the
beginning of the string, and may thus segfault.