Fixing buffer overflow issue in chat.c

Fixing buffer overflow issue in chat.c

There were two issues here, the report_buffer is too small to hold the
value, and accessing the memory outside its bounds. The following fixes
was made:
- Expand the size of report_buffer to 4096 from 256, this is to account
  for handling of really long GSM USSD report strings
- Make sure to not to access memory outside the bounds of the buffer

Signed-off-by: Robert Bartel <r.bartel@gmx.net>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>