kinetis: fix warning about malloc(0) w/assert

Change-Id: Ib40204675bfc5429c744f9ed7e2f7098384b753d
Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>
Reviewed-on: http://openocd.zylin.com/47
Tested-by: jenkins
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>

src/flash/nor/kinetis.c

@@ -467,6 +467,7 @@ static int kinetis_probe(struct flash_bank *bank)
 	}
 
 	bank->num_sectors = bank->size / (2 * 1024);
+	assert(bank->num_sectors > 0);
 	bank->sectors = malloc(sizeof(struct flash_sector) * bank->num_sectors);
 
 	for (i = 0; i < bank->num_sectors; i++) {