Fixed arg_formatter_base::write_pointer to not mutate the format specs.

This fixes cases where arg_formatters are reused, like with arg_join.

include/fmt/format.h

@@ -1431,9 +1431,10 @@ class arg_formatter_base {
   }
 
   void write_pointer(const void *p) {
-    specs_.flags_ = HASH_FLAG;
-    specs_.type_ = 'x';
-    writer_.write_int(reinterpret_cast<uintptr_t>(p), specs_);
+    format_specs specs = specs_;
+    specs.flags_ = HASH_FLAG;
+    specs.type_ = 'x';
+    writer_.write_int(reinterpret_cast<uintptr_t>(p), specs);
   }
 
  protected:

test/format-test.cc

@@ -1287,6 +1287,7 @@ TEST(FormatTest, JoinArg) {
   std::vector<float> v2;
   v2.push_back(1.2f);
   v2.push_back(3.4f);
+  void *v3[2] = { &v1[0], &v1[1] };
 
   EXPECT_EQ("(1, 2, 3)", format("({})", join(v1, v1 + 3, ", ")));
   EXPECT_EQ("(1)", format("({})", join(v1, v1 + 1, ", ")));
@@ -1298,6 +1299,9 @@ TEST(FormatTest, JoinArg) {
   EXPECT_EQ(L"(1, 2, 3)", format(L"({})", join(v1, v1 + 3, L", ")));
   EXPECT_EQ("1, 2, 3", format("{0:{1}}", join(v1, v1 + 3, ", "), 1));
 
+  EXPECT_EQ(format("{}, {}", v3[0], v3[1]),
+            format("{}", join(v3, v3 + 2, ", ")));
+
 #if FMT_USE_TRAILING_RETURN && (!FMT_GCC_VERSION || FMT_GCC_VERSION >= 405)
   EXPECT_EQ("(1, 2, 3)", format("({})", join(v1, ", ")));
   EXPECT_EQ("(+01.20, +03.40)", format("({:+06.2f})", join(v2, ", ")));