-[MGLMapView viewForAnnotation:] crashes when given annotation not on the map — map::at: key not found

[1ec5]

As reported in #6952 (comment), -[MGLMapView viewForAnnotation:] segfaults if you give it an MGLAnnotation that hasn’t been added to the MGLMapView or if you give it the user location annotation.

#1	0x000000010723788b in std::__1::map<id<MGLAnnotation> __strong, unsigned int, std::__1::less<id<MGLAnnotation> >, std::__1::allocator<std::__1::pair<objc_object<MGLAnnotation> const __strong, unsigned int> > >::at(std::__1::pair&) at /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/map:1578
#2	0x0000000107237755 in ::-[MGLMapView viewForAnnotation:](id) at /path/to/mapbox-gl-native-2/platform/ios/src/MGLMapView.mm:3104

/cc @boundsj

[1ec5]

This regression was introduced in #5987.

[boundsj]

This was fixed in #6957