Minor compiler compatibility fixes

- Initialize variable to avoid 'may be uninitialized' warning
- More reliable segfault (the previous version was compiled into an
undefined instruction by clang sometimes, thew new one is always a store
to zero).

src/V3LinkInc.cpp

@@ -176,7 +176,7 @@ private:
     void prepost_stmt_visit(AstNodeTriop* nodep) {
         iterateChildren(nodep);
 
-        AstNodeVarRef* varrefp;
+        AstNodeVarRef* varrefp = nullptr;
         if (m_unsupportedHere || !(varrefp = VN_CAST(nodep->rhsp(), VarRef))) {
             nodep->v3warn(E_UNSUPPORTED, "Unsupported: Incrementation in this context.");
             return;

src/V3Options.cpp

@@ -815,7 +815,7 @@ string V3Options::protectKeyDefaulted() {
 void V3Options::throwSigsegv() {  // LCOV_EXCL_START
 #if !(defined(VL_CPPCHECK) || defined(__clang_analyzer__))
     // clang-format off
-    { char* zp = nullptr; *zp = 0; }  // Intentional core dump, ignore warnings here
+    *static_cast<volatile char*>(nullptr) = 0;  // Intentional core dump, ignore warnings here
     // clang-format on
 #endif
 }  // LCOV_EXCL_STOP