[sysprof] stackstash: add assertions before silencing warnings

[Christian Hergert <chergert src gnome org>]

commit 8ed6af618b25b56bc9e210e4345a336eb962a6d4
Author: Christian Hergert <chergert redhat com>
Date:   Fri Apr 1 17:04:27 2022 -0700

    stackstash: add assertions before silencing warnings
    
    GCC warns about dangling pointers here, which is completely fine given
    how this code works.

 src/stackstash.c | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/src/stackstash.c b/src/stackstash.c
index 9b333598..f51737fe 100644
--- a/src/stackstash.c
+++ b/src/stackstash.c
@@ -273,7 +273,10 @@ do_callback (StackNode *node,
     StackLink link;
 
     if (trace)
+    {
+        g_assert (trace->prev == NULL);
         trace->prev = &link;
+    }
 
     link.next = trace;
     link.prev = NULL;
@@ -291,7 +294,10 @@ do_callback (StackNode *node,
     }
 
     if (trace)
+    {
+        g_assert (trace->prev == &link);
         trace->prev = NULL;
+    }
 }
 
 void