[sysprof] Protect against NULL sym string.

[Jeffrey Stedfast <fejj src gnome org>]

commit 9eb32f4340a079e497dca03de3ebb8f62a2cdcb5
Author: Jeffrey Stedfast <fejj gnome org>
Date:   Tue Dec 20 21:52:35 2011 -0500

    Protect against NULL sym string.

 tracker.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/tracker.c b/tracker.c
index dfdbec5..ec21e17 100644
--- a/tracker.c
+++ b/tracker.c
@@ -825,7 +825,7 @@ lookup_kernel_symbol (gulong address)
      * Below we filter out the [irq stack]
      */
     i = 0;
-    while (skip_kernel_symbols[i][0] != '\0')
+    while (sym && skip_kernel_symbols[i][0] != '\0')
     {
 	if (strcmp (sym, skip_kernel_symbols[i]) == 0)
 	{