fw_printenv: Fix crash due to incorrect size for malloc'ed string.

Using sizeof gives the size of the pointer only, not the string. This
could easily lead to crashes when using -l argument.

Signed-off-by: Kristian Amlie <kristian.amlie@northern.tech>
diff --git a/tools/env/fw_env_main.c b/tools/env/fw_env_main.c
index d93a915..fb4afa5 100644
--- a/tools/env/fw_env_main.c
+++ b/tools/env/fw_env_main.c
@@ -239,7 +239,7 @@
 	argv += optind;
 
 	if (env_opts.lockname) {
-		lockname = malloc(sizeof(env_opts.lockname) +
+		lockname = malloc(strlen(env_opts.lockname) +
 				sizeof(CMD_PRINTENV) + 10);
 		if (!lockname) {
 			fprintf(stderr, "Unable allocate memory");