libpmcstat: fix pmcstat -G on older-ish -CURRENT w/ drm-kmod

The linuxkpi_gplv2.ko module on older-ish -CURRENT had absolutely no
.text contents, but it was still loaded. Instead of hitting the later
assertion because 0 is not less than 0, we can just skip images like
this as we should not have any samples from them.

Reviewed by: gallatin

(cherry picked from commit 77721403c91d67dbfd5a2c5c667e7f5d87acb3f6)