LLVM backend: Remove bad assert()
lex: Sanitize chars for source lines emitted into .asm/.c under -g

When reading Unicode input, hReadChar() could end up passing chars > 255
to DZstrConcatAssignC() which is used to build up the lex.ctx->currline
which is used to emit the current source lines into the .asm/.c output
under -g for debugging purposes.

Since DZstrConcatAssignC() would only use the low byte of the given char,
as lex.ctx->currline is a zstring (and .asm/.c output files don't use
Unicode anyways), it truncated the Unicode char and ended up with only its
low byte, which could be anything 0..255, including the ASCII control
chars 0..31 some of which can't safely be emitted into comments in .asm/.c
files (newlines, end of transmission, etc.).