[PATCH] evaluate getenv() return value for udev_config.c

[PATCH] evaluate getenv() return value for udev_config.c

Small patch to fix the evaluation logic for the return value of getenv()
in udev_config.c file.  Basically, the actual values for the environment
variables "UDEV_NO_SLEEP" and "UDEV_NO_DEVD" were not being checked.
For example UDEV_NO_SLEEP could have been set to false but the line:

if (getenv("UDEV_NO_SLEEP") != NULL)

in this case would always evaluate to true, since getenv() returns
char*, thus the "udev_sleep" variable would be set incorrectly.  The
patch makes sure to check the value returned by getenv() not just if
getenv() returned a value.  Hope this made sense.  Thanks,