Don't dereference a null pointer

Fixes #564

I don't actually believe such a file can be created through normal means. I imagine this was created by a 3rd party tool, probably by some instrument manufacturer. But checking for a null pointer here doesn't seem to be actively harmful.

src/XlsWorkBook.h

@@ -90,6 +90,9 @@ class XlsWorkBook {
     if (n_formats > 0) {
       for (int i = 0; i < n_formats; ++i) {
         const xls::st_format::st_format_data format = pWB->formats.format[i];
+        if (format.value == nullptr) {
+          continue;
+        }
         // format.value = format string
         // in xlsx, this is formatCode
         std::string code((char*) format.value);