dyndbg: fix old BUG_ON in >control parser

Fix a BUG_ON from 2009.  Even if it looks "unreachable" (I didn't
really look), lets make sure by removing it, doing pr_err and return
-EINVAL instead.

Cc: stable <stable@kernel.org>
Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Link: https://lore.kernel.org/r/20240429193145.66543-2-jim.cromie@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>