better output when @cImport generates invalid zig

src/errmsg.cpp

@@ -123,6 +123,7 @@ ErrorMsg *err_msg_create_with_line(Buf *path, size_t line, size_t column,
     size_t end_line = line + 1;
     size_t line_end_offset = (end_line >= line_offsets->length) ? buf_len(source) : line_offsets->at(line + 1);
     size_t len = (line_end_offset + 1 > line_start_offset) ? (line_end_offset - line_start_offset - 1) : 0;
+    if (len == SIZE_MAX) len = 0;
 
     buf_init_from_mem(&err_msg->line_buf, buf_ptr(source) + line_start_offset, len);