clk: Fix potential NULL dereference in clk_fetch_parent_index()

Don't compare the parent clock name with a NULL name in the
clk_parent_map. This prevents a kernel crash when passing NULL
core->parents[i].name to strcmp().

An example which triggered this is a mux clock with four parents when
each of them is referenced in the clock driver using
clk_parent_data.fw_name and then calling clk_set_parent(clk, 3rd_parent)
on this mux.
In this case the first parent is also the HW default so
core->parents[i].hw is populated when the clock is registered. Calling
clk_set_parent(clk, 3rd_parent) will then go through all parents and
skip the first parent because it's hw pointer doesn't match. For the
second parent no hw pointer is cached yet and clk_core_get(core, 1)
returns a non-matching pointer (which is correct because we are comparing
the second with the third parent). Comparing the result of
clk_core_get(core, 2) with the requested parent gives a match. However
we don't reach this point because right after the clk_core_get(core, 1)
mismatch the old code tried to !strcmp(parent->name, NULL) (where the
second argument is actually core->parents[i].name, but that was never
populated by the clock driver).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Link: https://lkml.kernel.org/r/20190815223155.21384-1-martin.blumenstingl@googlemail.com
Fixes: fc0c209c14 ("clk: Allow parents to be specified without string names")
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

drivers/clk/clk.c

@@ -1654,7 +1654,8 @@ static int clk_fetch_parent_index(struct clk_core *core,
 			break;
 
 		/* Fallback to comparing globally unique names */
-		if (!strcmp(parent->name, core->parents[i].name))
+		if (core->parents[i].name &&
+		    !strcmp(parent->name, core->parents[i].name))
 			break;
 	}