coresight: Fix NULL pointer dereference in _coresight_build_path

_coresight_build_path assumes that all the connections of a csdev
has the child_dev initialised. This may not be true if the particular
component is not supported by the kernel config(e.g TPIU) but is
present in the DT. In which case, building a path can cause a crash like this :

  Unable to handle kernel NULL pointer dereference at virtual address 00000010
  pgd = ffffffc9750dd000
  [00000010] *pgd=00000009f5e90003, *pud=00000009f5e90003, *pmd=0000000000000000
  Internal error: Oops: 96000006 [#1] PREEMPT SMP
  Modules linked in:
  CPU: 4 PID: 1348 Comm: bash Not tainted 4.6.0-next-20160517 #1646
  Hardware name: ARM Juno development board (r0) (DT)
  task: ffffffc97517a280 ti: ffffffc9762c4000 task.ti: ffffffc9762c4000
  PC is at _coresight_build_path+0x18/0xe4
  LR is at _coresight_build_path+0xc0/0xe4
  pc : [<ffffff80083d5130>] lr : [<ffffff80083d51d8>] pstate: 20000145
  sp : ffffffc9762c7ba0

  [<ffffff80083d5130>] _coresight_build_path+0x18/0xe4
  [<ffffff80083d51d8>] _coresight_build_path+0xc0/0xe4
  [<ffffff80083d51d8>] _coresight_build_path+0xc0/0xe4
  [<ffffff80083d51d8>] _coresight_build_path+0xc0/0xe4
  [<ffffff80083d51d8>] _coresight_build_path+0xc0/0xe4
  [<ffffff80083d51d8>] _coresight_build_path+0xc0/0xe4
  [<ffffff80083d5cdc>] coresight_build_path+0x40/0x68
  [<ffffff80083d5e14>] coresight_enable+0x74/0x1bc
  [<ffffff80083d60a0>] enable_source_store+0x3c/0x6c
  [<ffffff800830b17c>] dev_attr_store+0x18/0x28
  [<ffffff80081ca9c4>] sysfs_kf_write+0x40/0x50
  [<ffffff80081c9e38>] kernfs_fop_write+0x140/0x1cc
  [<ffffff8008163ec8>] __vfs_write+0x28/0x110
  [<ffffff8008164bf0>] vfs_write+0xa0/0x174
  [<ffffff8008165d18>] SyS_write+0x44/0xa0
  [<ffffff8008084e70>] el0_svc_naked+0x24/0x28

Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hwtracing/coresight/coresight.c

@@ -385,7 +385,6 @@ static int _coresight_build_path(struct coresight_device *csdev,
 	int i;
 	bool found = false;
 	struct coresight_node *node;
-	struct coresight_connection *conn;
 
 	/* An activated sink has been found.  Enqueue the element */
 	if ((csdev->type == CORESIGHT_DEV_TYPE_SINK ||
@@ -394,8 +393,9 @@ static int _coresight_build_path(struct coresight_device *csdev,
 
 	/* Not a sink - recursively explore each port found on this element */
 	for (i = 0; i < csdev->nr_outport; i++) {
-		conn = &csdev->conns[i];
-		if (_coresight_build_path(conn->child_dev, path) == 0) {
+		struct coresight_device *child_dev = csdev->conns[i].child_dev;
+
+		if (child_dev && _coresight_build_path(child_dev, path) == 0) {
 			found = true;
 			break;
 		}