printenv: Correct out-of-memory condition check.

In common/cmd_nvedit.c, en env_print(), the wrong type is used for len.
hexport_r() returns -1 on error (like OOM), which is converted to
0xffffffff when put in an unsigned. Said value is obviously bigger then
0, and as a result an uninitialized string is then displayed. Other
usages of hexport_r() in the code correctly uses ssize_t to keep its
return value.

Signed-off-by: Maxime Larocque <maxmtl2002@yahoo.ca>