Fix null access on optimized-out fork statements (#3658)

`V3SchedTiming` currently assumes that if a fork still exists, it must
have statements within it (otherwise it would have been deleted by
`V3Timing`). However, in a case like this:
```
module t;
    reg a;
    initial fork a = 1; join
endmodule
```
the assignment in the fork is optimized out by `V3Dead` after
`V3Timing`. This leads to `V3SchedTiming` accessing fork's `stmtsp`
pointer, which at this point is null. This patch addresses that issue.

Signed-off-by: Krzysztof Bieganski <kbieganski@antmicro.com>

src/V3SchedTiming.cpp

@@ -323,8 +323,10 @@ void transformForks(AstNetlist* const netlistp) {
             iterateChildrenConst(nodep);  // Const, so we don't iterate the calls twice
             // Replace self with the function calls (no co_await, as we don't want the main
             // process to suspend whenever any of the children do)
-            nodep->replaceWith(nodep->stmtsp()->unlinkFrBackWithNext());
+            // V3Dead could have removed all statements from the fork, so guard against it
-            VL_DO_DANGLING(nodep->deleteTree(), nodep);
+            AstNode* const stmtsp = nodep->stmtsp();
+            if (stmtsp) nodep->addNextHere(stmtsp->unlinkFrBackWithNext());
+            VL_DO_DANGLING(nodep->unlinkFrBack()->deleteTree(), nodep);
         }
         void visit(AstBegin* nodep) override {
             UASSERT_OBJ(m_forkp, nodep, "Begin outside of a fork");

test_regress/t/t_timing_debug2.out

@@ -15,6 +15,8 @@
 -V{t#,#}+    Vt_timing_debug2___024root____Vfork___h########__0__5
 -V{t#,#}             Awaiting join of fork at: t/t_timing_fork_join.v:13
 -V{t#,#}+    Vt_timing_debug2___024root___eval_initial__TOP__1
+-V{t#,#}+    Vt_timing_debug2___024root___eval_initial__TOP__2
+-V{t#,#}             Awaiting join of fork at: t/t_timing_fork_join.v:83
 -V{t#,#}+    Vt_timing_debug2___024root___eval_settle
 -V{t#,#}+ Eval
 -V{t#,#}+    Vt_timing_debug2___024root___eval

test_regress/t/t_timing_fork_join.v

@@ -77,4 +77,8 @@ module t;
       $finish;
     end
     initial #100 $stop; // timeout
+
+    // Test optimized-out fork statements:
+    reg a;
+    initial fork a = 1; join
 endmodule