Add null RegisterContext assertions

This makes these failures slightly more obvious, avoiding the need to
run LLDB under a debugger or rely on a LLDB core.  I encountered these
while bringing up a new OS/arch combination.

llvm-svn: 230236
diff --git a/lldb/source/Target/ThreadPlan.cpp b/lldb/source/Target/ThreadPlan.cpp
index 2c9b7fc..f2d7a18 100644
--- a/lldb/source/Target/ThreadPlan.cpp
+++ b/lldb/source/Target/ThreadPlan.cpp
@@ -156,6 +156,7 @@
         if (log)
         {
             RegisterContext *reg_ctx = m_thread.GetRegisterContext().get();
+            assert (reg_ctx);
             addr_t pc = reg_ctx->GetPC();
             addr_t sp = reg_ctx->GetSP();
             addr_t fp = reg_ctx->GetFP();