dyndbg: fix use before null check

dyndbg: fix use before null check

In commit a2d375eda771 ("dyndbg: refine export, rename to
dynamic_debug_exec_queries()"), a string is copied before checking it
isn't NULL.  Fix this, report a usage/interface error, and return the
proper error code.

Fixes: a2d375eda771 ("dyndbg: refine export, rename to dynamic_debug_exec_queries()")
Cc: stable@vger.kernel.org
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-v2 drop comment tweak, improve commit message

Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Link: https://lore.kernel.org/r/20201209183625.2432329-1-jim.cromie@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>