Add check for nullptr dereference in process_port (#2687)

This PR fixes segmentation fault seen in this issue:
chipsalliance/Surelog#4017, but it doesn't fix
the actual problem with missing `vpiActual` in UHDM tree.

src/frontends/systemverilog/uhdm_ast.cc

@@ -4998,6 +4998,8 @@ void UhdmAst::process_port()
     std::vector<AST::AstNode *> unpacked_ranges; // comes after wire name
     if (lowConn_h) {
         vpiHandle actual_h = vpi_handle(vpiActual, lowConn_h);
+        if (!actual_h)
+            log_error("vpiPort doesn't contain vpiActual in it's subtree. Aborting.\n");
         auto actual_type = vpi_get(vpiType, actual_h);
         switch (actual_type) {
         case vpiModport: {