power: test_power: correctly handle empty writes

Writing 0 length data into test_power makes it access an invalid array
location and kill the system.

Fixes: f17ef9b2d ("power: Make test_power driver more dynamic.")
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/drivers/power/test_power.c b/drivers/power/test_power.c
index 83c42ea88f2b..57246cdbd042 100644
--- a/drivers/power/test_power.c
+++ b/drivers/power/test_power.c
@@ -301,6 +301,8 @@ static int map_get_value(struct battery_property_map *map, const char *key,
         buf[MAX_KEYLENGTH-1] = '\0';
 
         cr = strnlen(buf, MAX_KEYLENGTH) - 1;
+        if (cr < 0)
+                return def_val;
         if (buf[cr] == '\n')
                 buf[cr] = '\0';