Correctly reject bad tftp addresses earlier, rather than later.

Correctly reject bad tftp addresses earlier, rather than later.

This check is for end == NULL but was meant to be *end == '\0'.  Without
this change, we'll pass a plausibly bad address (i.e. one with no ']' at
the end) to Mtftp(... READ_FILE ...), which should fail correctly, but
our error messaging will be inconsistent.

Signed-off-by: Peter Jones <pjones@redhat.com>