ata: libahci_platform: ahci_platform_find_clk: oops, NULL pointer

When booting a arm 32-bit kernel with config CONFIG_AHCI_DWC enabled on
a am57xx-evm board. This happens when the clock references are unnamed
in DT, the strcmp() produces a NULL pointer dereference, see the
following oops, NULL pointer dereference:

[    4.673950] Unable to handle kernel NULL pointer dereference at virtual address 00000000
[    4.682098] [00000000] *pgd=00000000
[    4.685699] Internal error: Oops: 5 [#1] SMP ARM
[    4.690338] Modules linked in:
[    4.693420] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.1.0-rc7 #1
[    4.699615] Hardware name: Generic DRA74X (Flattened Device Tree)
[    4.705749] PC is at strcmp+0x0/0x34
[    4.709350] LR is at ahci_platform_find_clk+0x3c/0x5c
[    4.714416] pc : [<c130c494>]    lr : [<c0c230e0>]    psr: 20000013
[    4.720703] sp : f000dda8  ip : 00000001  fp : c29b1840
[    4.725952] r10: 00000020  r9 : c1b23380  r8 : c1b23368
[    4.731201] r7 : c1ab4cc4  r6 : 00000001  r5 : c3c66040  r4 : 00000000
[    4.737762] r3 : 00000080  r2 : 00000080  r1 : c1ab4cc4  r0 : 00000000
[...]
[    4.998870]  strcmp from ahci_platform_find_clk+0x3c/0x5c
[    5.004302]  ahci_platform_find_clk from ahci_dwc_probe+0x1f0/0x54c
[    5.010589]  ahci_dwc_probe from platform_probe+0x64/0xc0
[    5.016021]  platform_probe from really_probe+0xe8/0x41c
[    5.021362]  really_probe from __driver_probe_device+0xa4/0x204
[    5.027313]  __driver_probe_device from driver_probe_device+0x38/0xc8
[    5.033782]  driver_probe_device from __driver_attach+0xb4/0x1ec
[    5.039825]  __driver_attach from bus_for_each_dev+0x78/0xb8
[    5.045532]  bus_for_each_dev from bus_add_driver+0x17c/0x220
[    5.051300]  bus_add_driver from driver_register+0x90/0x124
[    5.056915]  driver_register from do_one_initcall+0x48/0x1e8
[    5.062591]  do_one_initcall from kernel_init_freeable+0x1cc/0x234
[    5.068817]  kernel_init_freeable from kernel_init+0x20/0x13c
[    5.074584]  kernel_init from ret_from_fork+0x14/0x2c
[    5.079681] Exception stack(0xf000dfb0 to 0xf000dff8)
[    5.084747] dfa0:                                     00000000 00000000 00000000 00000000
[    5.092956] dfc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
[    5.101165] dfe0: 00000000 00000000 00000000 00000000 00000013 00000000
[    5.107818] Code: e5e32001 e3520000 1afffffb e12fff1e (e4d03001)
[    5.114013] ---[ end trace 0000000000000000 ]---

Add an extra check in the if-statement if hpriv-clks[i].id.

Fixes: 6ce73f3a ("ata: libahci_platform: Add function returning a clock-handle by id")
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>

drivers/ata/libahci_platform.c

@@ -109,7 +109,7 @@ struct clk *ahci_platform_find_clk(struct ahci_host_priv *hpriv, const char *con
 	int i;
 
 	for (i = 0; i < hpriv->n_clks; i++) {
-		if (!strcmp(hpriv->clks[i].id, con_id))
+		if (hpriv->clks[i].id && !strcmp(hpriv->clks[i].id, con_id))
 			return hpriv->clks[i].clk;
 	}