`eb_is_text_stopped`: Fix crash when built with `--enable-pthread`

[mvf]

This fixes a crash when built with --enable-pthread. eb_is_text_stopped() calls eb_lock() on the book struct itself, instead of its lock member. Compiler warns too:

readtext.c: In function ‘eb_is_text_stopped’:
readtext.c:1563:13: warning: passing argument 1 of ‘eb_lock’ from incompatible pointer type [-Wincompatible-pointer-types]
 1563 |     eb_lock(book);
      |             ^~~~
      |             |
      |             EB_Book * {aka struct EB_Book_Struct *}
In file included from readtext.c:33:
build-post.h:277:23: note: expected ‘EB_Lock *’ {aka ‘struct EB_Lock_Struct *’} but argument is of type ‘EB_Book *’ {aka ‘struct EB_Book_Struct *’}
277 | void eb_lock(EB_Lock *lock);
    |              ~~~~~~~~~^~~~