Fix CI: image tests with GCC on Linux, without exceptions, and on clang 3.6

- ci_test_gcc (Linux x86-64): static_cast<std::size_t>(header_field(...))
  was a useless cast (std::uint64_t is std::size_t there), and returning
  std::mt19937::result_type (std::uint_fast32_t, unsigned long there) as
  std::uint32_t failed -Werror=conversion. Cast named variables instead.
- ci_test_noexceptions: the error, check, and damaged-image tests test the
  exceptions of load() and save() and catch outside a CHECK_THROWS, which
  aborts with JSON_NOEXCEPTION; compile them and their helpers only with
  exceptions.
- clang 3.6: value-initialize a const json_document (no user-provided
  default constructor, CWG 253).
- Format the image fuzzer with the pinned astyle, which the "check" job
  runs over tests/.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/fuzzer-json_view_image.cpp

@@ -41,7 +41,10 @@ using image_check = json_document::image_check;
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
 {
     // the input as an image
-    for (const image_check check : {image_check::full, image_check::bounds})
+    for (const image_check check :
+            {
+                image_check::full, image_check::bounds
+            })
     {
         json_document d;
         try
@@ -79,7 +82,10 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
     if (!parsed.is_discarded())
     {
         const std::vector<std::uint8_t> image = parsed.save();
-        for (const image_check check : {image_check::full, image_check::bounds, image_check::none})
+        for (const image_check check :
+                {
+                    image_check::full, image_check::bounds, image_check::none
+                })
         {
             const json_document loaded = json_document::load(image, check);
             assert(loaded.root().dump() == parsed.root().dump());