cogl/trace: Add check for failed sysprof context creation

This can fail for example when passing an invalid filename to
mutter --profile, which leads to assertion failures down the line.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3397>

cogl/cogl/cogl-trace.c

@@ -102,6 +102,9 @@ cogl_trace_context_new (int         fd,
       writer = sysprof_capture_writer_new (COGL_TRACE_OUTPUT_FILE, BUFFER_LENGTH);
     }
 
+  if (!writer)
+    return NULL;
+
   context = g_new0 (CoglTraceContext, 1);
   context->writer = writer;
   g_atomic_ref_count_init (&context->ref_count);
@@ -144,6 +147,13 @@ setup_trace_context (int          fd,
 
   cogl_trace_context = cogl_trace_context_new (fd, filename);
 
+  if (!cogl_trace_context)
+    {
+      g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
+                   "Failed to setup trace context");
+      return FALSE;
+    }
+
   return TRUE;
 }