_scanf.c: Implement 'm' modifier for 'c' and '[' conversions.

The current code implements the 'm' modifier only for 's'
conversions and would cause a segfault if it was used for 'c'
or '[' conversions. This patch extends the code to cover these
cases too.

Signed-off-by: Will Newton <will.newton@imgtec.com>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>