convertbmp: detect invalid file dimensions early

width/length dimensions read from bmp headers are not necessarily
valid. For instance they may have been maliciously set to very large
values with the intention to cause DoS (large memory allocation, stack
overflow). In these cases we want to detect the invalid size as early
as possible.

This commit introduces a counter which verifies that the number of
written bytes corresponds to the advertized width/length.

Fixes #1059 (CVE-2018-6616).