fix a use-of-unitialized variable bug that is triggered by cold external cache and a command line such as this:
fix a use-of-unitialized variable bug that is triggered by cold external cache
and a command line such as this:

souper  -stp-path=/usr/local/bin/stp -souper-no-infer -souper-external-cache 9gLRvwcI9i.bc

chubbymaggie/souper,google/souper,jubitaneja/souper,jubitaneja/souper,chenyang78/souper,jubitaneja/souper,regehr/souper,lazyparser/souper,regehr/souper,lazyparser/souper,rsas/souper,chubbymaggie/souper,regehr/souper,chubbymaggie/souper,rsas/souper,google/souper,lazyparser/souper,regehr/souper,rsas/souper,chubbymaggie/souper,jubitaneja/souper,chenyang78/souper,lazyparser/souper,google/souper,google/souper,chubbymaggie/souper,lazyparser/souper,jubitaneja/souper,regehr/souper,rsas/souper,google/souper,chenyang78/souper,chenyang78/souper,chenyang78/souper,rsas/souper,chenyang78/souper,jubitaneja/souper,rsas/souper