Add null check for character class.  Previously, an inverted character
class could result in a nul byte being matched and lead to the function
reading past the end of the inputted string.

Link: https://lkml.kernel.org/r/20240826155709.12383-1-swaminathanalok@gmail.comSigned-off-by: Alok Swaminathan <swaminathanalok@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>