stage2: tokenizer: require null terminated source

By requiring the source file to be null-terminated, we avoid extra
branching while simplifying the logic at the same time.

Running ast-check on a large zig source file (udivmodti4_test.zig),
master branch compared to this commit:
 * 4% faster wall clock
 * 7% fewer cache misses
 * 1% fewer branches

src/Compilation.zig

@@ -1541,7 +1541,7 @@ pub fn create(gpa: *Allocator, options: InitOptions) !*Compilation {
         }
     }
 
-    if (comp.bin_file.options.use_stage1) {
+    if (comp.bin_file.options.use_stage1 and comp.bin_file.options.module != null) {
         try comp.work_queue.writeItem(.{ .stage1_module = {} });
     }