Fix UB introduced by copying detail::FormatListN objects. (#59)

When copying a detail::FormatListN object, the m_formatterStore must also be copied, otherwise the pointer in the parent FormatList class are possibly invalid.

This addresses the segfault explained in #58. The issue arises when makeFormatList returns an object which holds a pointer to memory created for the temporary object on the stack.

Author: dakep

tinyformat.h

@@ -984,6 +984,10 @@ class FormatListN : public FormatList
         TINYFORMAT_FOREACH_ARGNUM(TINYFORMAT_MAKE_FORMATLIST_CONSTRUCTOR)
 #       undef TINYFORMAT_MAKE_FORMATLIST_CONSTRUCTOR
 #endif
+        FormatListN(const FormatListN& other)
+            : FormatList(&m_formatterStore[0], N)
+        { std::copy(&other.m_formatterStore[0], &other.m_formatterStore[N],
+                    &m_formatterStore[0]); }
 
     private:
         FormatArg m_formatterStore[N];