Force null pointer exception for code outside of a function

Force null pointer exception for code outside of a function

TinyCC sometimes has problems to evaluate constant expressions at
compile time. This leads to code being generated outside of functions.
Without this patch some of these bugs are not caught, because
cur_text_section still points to the section of the last function.
Before the first function cur_text_section is uninitialized.
Setting cur_text_section to a null pointer should make TinyCC die in
all cases.