Reject input that contains null characters

Null characters in the middle of strings interact badly with the RGBDS
codebase, which assumes null-terminated strings. There is no reason to
support null characters in input source code, so the simplest way to deal
with null characters is to reject them early.

test/asm/null-in-macro.out

@@ -1,2 +1,2 @@
-ERROR: null-in-macro.asm(1):
-    Unterminated MACRO definition.
+ERROR: null-in-macro.asm(2):
+    Found null character