Fix crash (bad stack) when calling coroutines with efun::trace()

When efun::trace() is active, the stack pointer was modified in a wrong
way when executing a yield() statement.

src/interpret.c

@@ -10042,6 +10042,8 @@ eval_instruction (bytecode_p first_instruction
         sp = restore_argument_frames(sp-1, &ap)+1;
         if (sp < inter_sp)
             *sp = *inter_sp;
+
+        tracedepth++;
     }
     SET_TRACE_EXEC();
 
@@ -11389,7 +11391,7 @@ again:
 
         tracedepth--; /* We leave this level */
         if (trace_level)
-            do_trace_return(sp);
+            do_trace_return(inter_sp);
 
         pop_control_stack();
 

test/t-trace-coroutine.c

@@ -0,0 +1,60 @@
+#include "/inc/base.inc"
+#include "/inc/client.inc"
+
+#include "/sys/trace.h"
+
+/* We test calling coroutines while efun::trace() is active. */
+
+void run_server()
+{
+    efun::trace(TRACE_CALL_OTHER|TRACE_RETURN|TRACE_ARGS);
+
+    coroutine cr = async function void()
+    {
+        yield("A");
+        yield("B");
+        yield("C");
+    };
+
+    foreach(string str: cr)
+        if (!stringp(str))
+        {
+            efun::shutdown(1);
+            return;
+        }
+
+    efun::trace(TRACE_NOTHING);
+
+    /* The test succeeds, if we didn't crash. */
+    call_out(#'efun::shutdown,0,0);
+}
+
+void run_client()
+{
+    input_to("client_input");
+}
+
+void client_input(string str)
+{
+    input_to("client_input");
+}
+
+void run_test()
+{
+    msg("\nRunning test for tracing with coroutines:\n"
+          "-----------------------------------------\n");
+
+    /* For efun::trace() we need an interactive object. */
+    connect_self("run_server", "run_client");
+}
+
+string *epilog(int eflag)
+{
+    run_test();
+    return 0;
+}
+
+int valid_trace(string what, int|string arg)
+{
+    return 1;
+}