bugfix: don't underflow for thousands separator for things with less than three digits to begin with.

src/bignum_helper.cpp

@@ -154,7 +154,7 @@ std::string apply(number_format const & fmt, std::string text) {
   auto decimal_place = text.find('.');
   if (fmt.separate_thousands) {
     constexpr std::size_t const LEN_THOUSAND{3};
-    auto pos = std::min(text.size(), decimal_place);
+    auto pos = std::max(std::min(text.size(), decimal_place), LEN_THOUSAND);
     for (pos -= LEN_THOUSAND; pos > LEN_THOUSAND; pos -= LEN_THOUSAND) {
       text.insert(pos, ",");
     }