Fix heap-use-after-free in VlRNG::VlRNG() due to dangling thread-local t_currentp

When a function (e.g. eval_static) creates a VlProcess via make_shared
and sets VlProcess::currentp(vlProcess.get()), the thread-local
t_currentp pointer is left dangling after the shared_ptr destroys the
VlProcess object at function exit. A subsequent function (e.g.
eval_initial) that constructs a new VlProcess triggers VlRNG::VlRNG(),
which calls VlProcess::currentRng() and reads the freed t_currentp,
causing a heap-use-after-free.

Fix by clearing t_currentp in ~VlProcess() when it points to the
dying object, redirecting to the parent process if one exists.

docs/CONTRIBUTORS

@@ -61,6 +61,7 @@ David Turner
 Dercury
 Diego Roux
 Dominick Grochowina
+Dragon-Git
 Don Williamson
 Drew Ranck
 Drew Taussig

include/verilated_types.h

@@ -337,6 +337,7 @@ public:
 
     ~VlProcess() {
         if (m_parentp) m_parentp->detach(this);
+        if (t_currentp == this) t_currentp = m_parentp;
     }
 
     void attach(VlProcess* childp) { m_children.insert(childp); }