pinctrl: core: make sure strcmp() doesn't get a null parameter

Some drivers, for example, QCOM's qdf2xxx, set groups[gpio].name only
when gpio is valid, and leave invalid gpio names as null.
If we want to access the sys node "pinconf-groups",
pinctrl_get_group_selector() -> get_group_name() may return a null
pointer if group_selector is invalid, then the below Kernel panic
would happen since strcmp() uses this null pointer to do comparison.

 Unable to handle kernel NULL pointer dereference at ss 00000000
el:Internal error: Oops: 9600000[ 143.080279]
SMP
 CPU: 19 PID: 2493 Comm: read_all Tainted: G O
.aarch64 #1
 Hardware name: HXT Semiconductor HXT REP-2 System
 PC is at strcmp+0x18/0x154
 LR is at pinctrl_get_group_selector+0x6c/0xe8
 Process read_all (pid: 2493, stack limit =
 Call trace:
 Exception stack
  strcmp+0x18/0x154
  pin_config_group_get+0x64/0xd8
  pinconf_generic_dump_one+0xd8/0x1c0
  pinconf_generic_dump_pins+0x94/0xc8
  pinconf_groups_show+0xb4/0x104
  seq_read+0x178/0x464
  full_proxy_read+0x6c/0xac
  __vfs_read+0x58/0x178
  vfs_read+0x94/0x164
  SyS_read+0x60/0xc0
  __sys_trace_return+0x0/0x4
 --[ end trace]--
 Kernel panic - not syncing: Fatal exception
Signed-off-by: Yanjiang Jin <yanjiang.jin@hxt-semitech.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/core.c

@@ -627,7 +627,7 @@ static int pinctrl_generic_group_name_to_selector(struct pinctrl_dev *pctldev,
 	while (selector < ngroups) {
 		const char *gname = ops->get_group_name(pctldev, selector);
 
-		if (!strcmp(function, gname))
+		if (gname && !strcmp(function, gname))
 			return selector;
 
 		selector++;
@@ -743,7 +743,7 @@ int pinctrl_get_group_selector(struct pinctrl_dev *pctldev,
 	while (group_selector < ngroups) {
 		const char *gname = pctlops->get_group_name(pctldev,
 							    group_selector);
-		if (!strcmp(gname, pin_group)) {
+		if (gname && !strcmp(gname, pin_group)) {
 			dev_dbg(pctldev->dev,
 				"found group selector %u for %s\n",
 				group_selector,