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.

src/asm/lexer.c

@@ -269,6 +269,7 @@ YY_BUFFER_STATE yy_create_buffer(FILE *f)
 	/* Convert all line endings to LF and spaces */
 
 	char *mem = pBuffer->pBuffer;
+	int32_t lineCount = 0;
 
 	while (*mem) {
 		if ((mem[0] == '\\') && (mem[1] == '\"' || mem[1] == '\\')) {
@@ -279,15 +280,22 @@ YY_BUFFER_STATE yy_create_buffer(FILE *f)
 			 || ((mem[0] == '\r') && (mem[1] == '\n'))) {
 				*mem++ = ' ';
 				*mem++ = '\n';
+				lineCount++;
 			/* LF and CR */
 			} else if ((mem[0] == '\n') || (mem[0] == '\r')) {
 				*mem++ = '\n';
+				lineCount++;
 			} else {
 				mem++;
 			}
 		}
 	}
 
+	if (mem != pBuffer->pBuffer + size) {
+		nLineNo = lineCount + 1;
+		fatalerror("Found null character");
+	}
+
 	/* Remove comments */
 
 	mem = pBuffer->pBuffer;

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