Fix bad_format_imginfo

The previous behavior could segfault with an imginfo string such as "%s%s %lux%lu", whereas this correctly rejects that format string.

The segfault was exposed in a number of programs that call out to rrdtool, and can occasionally cause a denial of service condition for them as well.

If it's feasible for you to cut a new release given that bad_format_imginfo doesn't exist at all in 1.4.8, that would be helpful. The lack of bad_format_imginfo can be a more exploitable security concern, while this change is "just" a denial of service. (This pull request still addresses a security concern, but it's less important than the raw format string being exposed.)