Crash in osgEarth::Annotation::PlaceNode::init due to call to __dynamic_cast with nullptr argument

Starting with gcc 6.x, some optimizations result in omission of null pointer checks in some places. This affects the osgearth package on Ubuntu 18.04, where osgEarth::Annotation::PlaceNode::init crashes on calling __dynamic_cast with null `this` pointer passed as argument.

The full test case reproducing the problem:

// -------- begin -----------
#include <fstream>
#include <osgEarthAnnotation/PlaceNode>

int main(int argc, char** argv)
{
    const auto earthFileName="/tmp/test.earth";
    std::ofstream earthFile(earthFileName);
    const char data[]="<map><options><terrain><color>#555555ff</color></terrain></options></map>";
    earthFile.write(data, sizeof data-1);
    earthFile.close();
    const auto earthNode = osgDB::readNodeFile(earthFileName);
    if(!earthNode)
    {
        OE_WARN << "Unable to load earth model \"" << earthFileName << "\"\n";
        return EXIT_FAILURE;
    }

    const auto mapNode = osgEarth::MapNode::findMapNode(earthNode);
    new osgEarth::Annotation::PlaceNode(mapNode, osgEarth::GeoPoint(mapNode->getMapSRS(), 37, 55, 0), "Icon on map");
}
// --------- end --------------

Backtrace from the SIGSEGV looks like this:

#0 0x00007ffff661b35d in __dynamic_cast () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#1 0x00007ffff6cfad6f in osgEarth::Annotation::PlaceNode::init (this=this@entry=0x5555559a5d30)
    at ./src/osgEarthAnnotation/PlaceNode.cpp:157
#2 0x00007ffff6cfc694 in osgEarth::Annotation::PlaceNode::PlaceNode (this=0x5555559a5d30, mapNode=<optimized out>, position=...,
    text=..., style=...) at ./src/osgEarthAnnotation/PlaceNode.cpp:85
#3 0x00005555555563f7 in main (argc=1, argv=0x7fffffffe1f8) at /home/ruslan/bak/osgearth-dynamic_cast-crash-demo.cpp:19

Here, if you look at the `instance` pointer variable in the frame #1, it appears equal 0x0. With this, `instance->asIcon()` has been called with `this==nullptr`. `asIcon` function does a `dynamic_cast`, which would check for null pointer and avoid calling `__dynamic_cast`, but due to an optimization relying on `this==nullptr` having Undefined Behavior, this check is omitted.

One easy way to "fix" this package is to use `-fno-delete-null-pointer-checks` g++ option when building osgEarth.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: libosgearthannotation5 2.9.0+dfsg-1
ProcVersionSignature: Ubuntu 4.15.0-36.39-generic 4.15.18
Uname: Linux 4.15.0-36-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.4
Architecture: amd64
Date: Mon Nov 19 13:32:55 2018
InstallationDate: Installed on 2015-04-22 (1307 days ago)
InstallationMedia: Kubuntu 14.04.2 LTS "Trusty Tahr" - Release amd64 (20150219.1)
SourcePackage: osgearth
UpgradeStatus: Upgraded to bionic on 2018-10-09 (40 days ago)