Avoid null-change KVO notifications

[1ec5]

If no access token is set in the Info.plist, avoid triggering a KVO notification about the access token changing. By the same token (heh), avoid pulling a C string out of the change dictionary if the access token is being nilled out.

Fixes a bug introduced in 90a50c0 for #1553.

/cc @incanus

[incanus]

Now I get a crash in nornmalizeURL() if I don't set a token, with no message.

#1  0x000000010a9dbbd7 in mbgl::util::mapbox::normalizeURL(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) ()
#2  0x000000010a9dbd14 in mbgl::util::mapbox::normalizeSourceURL(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) ()
#3  0x000000010a980e3b in mbgl::DefaultFileSource::request(mbgl::Resource const&, uv_loop_s*, std::__1::function<void (mbgl::Response const&)>) ()
#4  0x000000010a9316df in mbgl::Environment::request(mbgl::Resource const&, std::__1::function<void (mbgl::Response const&)>) ()
#5  0x000000010a949cbe in mbgl::Source::load() ()
#6  0x000000010a993f53 in mbgl::Style::Style(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, uv_loop_s*, mbgl::Environment&) ()
#7  0x000000010a940c6a in mbgl::MapContext::loadStyleJSON(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) ()
#8  0x000000010a984ce2 in mbgl::Request::notifyCallback() ()
#9  0x000000010aa1a5ae in uv__async_event at /Users/kkaefer/Code/mason/libuv-0.10.28/mason_packages/.build/libuv-0.10.28/src/unix/async.c:80
#10 0x000000010aa1a998 in uv__async_io at /Users/kkaefer/Code/mason/libuv-0.10.28/mason_packages/.build/libuv-0.10.28/src/unix/async.c:156
#11 0x000000010aa363ae in uv__io_poll at /Users/kkaefer/Code/mason/libuv-0.10.28/mason_packages/.build/libuv-0.10.28/src/unix/kqueue.c:233
#12 0x000000010aa1b0a3 in uv_run at /Users/kkaefer/Code/mason/libuv-0.10.28/mason_packages/.build/libuv-0.10.28/src/unix/core.c:317
#13 0x000000010a93ff83 in void mbgl::util::Thread<mbgl::MapContext>::run<std::__1::tuple<mbgl::View&, mbgl::FileSource&, mbgl::MapData&>, 0ul, 1ul, 2ul>(std::__1::tuple<mbgl::View&, mbgl::FileSource&, mbgl::MapData&>&&, std::__1::integer_sequence<unsigned long, 0ul, 1ul, 2ul>) ()
#14 0x000000010a93feaf in std::__1::__thread_proxy<std::__1::tuple<mbgl::util::Thread<mbgl::MapContext>::Thread<mbgl::View&, mbgl::FileSource&, mbgl::MapData&>(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, mbgl::util::ThreadPriority, mbgl::View&&&, mbgl::FileSource&&&, mbgl::MapData&&&)::'lambda'()> >(void*, void*) ()
#15 0x000000010e8a2268 in _pthread_body ()
#16 0x000000010e8a21e5 in _pthread_start ()
#17 0x000000010e8a041d in thread_start ()

Still testing as per #1746 (comment).

[1ec5]

@incanus, that is the intended location of the assertion. You’ve probably got an exception breakpoint; the message can’t be printed until you continue.

[1ec5]

We shouldn’t throw an exception up at the Cocoa level because there may be legitimate reasons for not specifying an access token, namely that the app is using a local style and non-Mapbox-hosted source.

[incanus]

Doh, yep. Looking good with the stripped library and exceptions:

libc++abi.dylib: terminating with uncaught exception of type std::runtime_error: You must provide a Mapbox API access token for Mapbox tile sources