fix compiler warnings in public header files

core.h: buffer had virtual functions but the dtor was not virtual.

format.f: fix two sign conversions warnings

include/fmt/core.h

@@ -669,7 +669,7 @@ template <typename T> class buffer {
         size_(sz),
         capacity_(cap) {}
 
-  ~buffer() = default;
+  virtual ~buffer() = default;
 
   /** Sets the buffer data and capacity. */
   void set(T* buf_data, size_t buf_capacity) FMT_NOEXCEPT {

include/fmt/format.h

@@ -1609,7 +1609,7 @@ template <typename OutputIt, typename Char, typename UInt> struct int_writer {
     char digits[40];
     format_decimal(digits, abs_value, num_digits);
     basic_memory_buffer<Char> buffer;
-    size += prefix_size;
+    size += static_cast<int>(prefix_size);
     const auto usize = to_unsigned(size);
     buffer.resize(usize);
     basic_string_view<Char> s(&sep, sep_size);
@@ -3493,7 +3493,7 @@ inline std::string to_string(T value) {
   // The buffer should be large enough to store the number including the sign or
   // "false" for bool.
   constexpr int max_size = detail::digits10<T>() + 2;
-  char buffer[max_size > 5 ? max_size : 5];
+  char buffer[max_size > 5 ? static_cast<size_t>(max_size) : 5];
   char* begin = buffer;
   return std::string(begin, detail::write<char>(begin, value));
 }