When extracting the capture buffer from a crashdump, only read the valid
portion of the capture buffer (db_capture_bufoff vs db_capture_bufsize).
This could result in outputting garbage (e.g. lots of 'p' characters if
DIAGNOSTIC is enabled) after the end of the capture buffer. While here,
fix a spelling nit.

Reported by: Mikolaj Golub to my trociny of gmail
MFC after: 3 days