Fix bug in crash handler register dump

This commit fixes a NULL-dereference in the register dump code for the
kernel crash handler. The task pointer can be NULL, in which case we
should not attempt to dump registers.

Change-Id: I3433ca466f34827f102d64ef2aac4fffc5b49b70
Reviewed-on: http://rchgit01.rchland.ibm.com/gerrit1/87422
Tested-by: Jenkins Server <pfd-jenkins+hostboot@us.ibm.com>
Tested-by: Jenkins OP Build CI <op-jenkins+hostboot@us.ibm.com>
Tested-by: Jenkins OP HW <op-hw-jenkins+hostboot@us.ibm.com>
Tested-by: FSP CI Jenkins <fsp-CI-jenkins+hostboot@us.ibm.com>
Reviewed-by: William G Hoffa <wghoffa@us.ibm.com>
Reviewed-by: Nicholas E Bofferding <bofferdn@us.ibm.com>
Reviewed-by: Daniel M Crowell <dcrowell@us.ibm.com>

src/kernel/misc.C

@@ -644,21 +644,22 @@ namespace KernelMisc
             l_frame = reinterpret_cast<uint64_t*>(*l_frame);
         }
 
-        printk("\n  GPRs for %d:\n", l_tid);
-
-        for (int i = 0; i < 16; ++i)
+        if (i_task)
         {
-            printk("    r%-2d = 0x%016lx    r%-2d = 0x%016lx\n",
-                   i,
-                   i_task->context.gprs[i],
-                   i + 16,
-                   i_task->context.gprs[i + 16]);
+            printk("\n  GPRs for %d:\n", l_tid);
+
+            for (int i = 0; i < 16; ++i)
+            {
+                printk("    r%-2d = 0x%016lx    r%-2d = 0x%016lx\n",
+                       i,
+                       i_task->context.gprs[i],
+                       i + 16,
+                       i_task->context.gprs[i + 16]);
+            }
         }
 
         printk("\n");
     }
-
-
 };
 
 namespace KernelMemState