staging: tidspbridge/pmgr: additional checking after return from strlen_user

  strlen_user will return the length including final NUL.
    and will return 0 if failed (for example: if user string not NUL terminated)

  so need check whether it is an invalid parameter.

addtional info:
  can reference the comments of strlen_user in lib/strnlen_user.c

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Cc: Omar Ramirez Luna <omar.ramirez@copitl.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>