Use a heuristic to detect empty strftime result (#367)

fmt/time.h

@@ -36,6 +36,13 @@ void format(BasicFormatter<char, ArgFormatter> &f,
       buffer.resize(start + count);
       break;
     }
+    if (size >= format.size() * 256) {
+      // If the buffer is 256 times larger than the format string, assume
+      // that `strftime` gives an empty result. There doesn't seem to be a
+      // better way to distinguish the two cases:
+      // https://github.com/fmtlib/fmt/issues/367
+      break;
+    }
     const std::size_t MIN_GROWTH = 10;
     buffer.reserve(buffer.capacity() + (size > MIN_GROWTH ? size : MIN_GROWTH));
   }

test/time-test.cc

@@ -27,3 +27,7 @@ TEST(TimeTest, GrowBuffer) {
   std::time_t t = std::time(0);
   fmt::format(s, *std::localtime(&t));
 }
+
+TEST(TimeTest, EmptyResult) {
+  EXPECT_EQ("", fmt::format("{}", std::tm()));
+}