Update: Improve design in f_conversion to be safer when handling variables allowed...

Update: Improve design in f_conversion to be safer when handling variables allowed to be replaced.

The constants are allowed (and encouraged) to be changed as desired by some developer or distributor.
This means constant strings like f_string_ascii_1_s could, in theory, be any length.

Change the do..while loops into normal while loops.
Change the while loop using sizeof(f_char_t) to instead use the actual constant string structure(via a pointer).

This situations was identified by the -fanalyzer functionality of GCC-12.1.
The -fanalyzer from earlier GCC versions, such as GCC 11, did not identify this.

The sizeof(f_char_t) is not strictly needed for the fwrite_unlocked() calls because they are wrapped in a loop.
The sizeof(f_char_t) can be assumed to be 1 in general and even if it is greater than 1, the loop will still ensure success.
Removing the sizeof(f_char_t) simplifies the design.