kv: fit string (including null) into given buffer

The old initializer implicitly truncated the null terminator of the
string, which is now a warning/error.

Author: ppannuto

examples/tests/kv/kv_system/main.c

@@ -8,7 +8,7 @@
 #define DATA_LEN 32
 
 uint8_t key_buf[KEY_LEN]   = "First Key";
-uint8_t data_buf[DATA_LEN] = "My secret key, that no one knows";
+uint8_t data_buf[DATA_LEN] = "My secret key that no one knows";
 uint8_t out_buf[DATA_LEN]  = "Just junk";
 
 static void kv_cb(int                          result,