checkout::crlf test: don't crash when no idx entry
checkout::crlf test: don't crash when no idx entry

When there's no matching index entry (for whatever reason), don't
try to dereference the null return value to get at the id.

Otherwise when we break something in the index API, the checkout
test crashes for confusing reasons and causes us to step through
it in a debugger thinking that we had broken much more than we
actually did.

Corillian/libgit2,yongthecoder/libgit2,yongthecoder/libgit2,Tousiph/Demo1,leoyanggit/libgit2,saurabhsuniljain/libgit2,Corillian/libgit2,leoyanggit/libgit2,stewid/libgit2,leoyanggit/libgit2,magnus98/TEST,leoyanggit/libgit2,yongthecoder/libgit2,jeffhostetler/public_libgit2,jeffhostetler/public_libgit2,magnus98/TEST,stewid/libgit2,KTXSoftware/libgit2,KTXSoftware/libgit2,stewid/libgit2,saurabhsuniljain/libgit2,KTXSoftware/libgit2,saurabhsuniljain/libgit2,joshtriplett/libgit2,yongthecoder/libgit2,jeffhostetler/public_libgit2,Corillian/libgit2,KTXSoftware/libgit2,saurabhsuniljain/libgit2,magnus98/TEST,yongthecoder/libgit2,yongthecoder/libgit2,KTXSoftware/libgit2,Tousiph/Demo1,magnus98/TEST,leoyanggit/libgit2,jeffhostetler/public_libgit2,Corillian/libgit2,leoyanggit/libgit2,stewid/libgit2,Tousiph/Demo1,joshtriplett/libgit2,joshtriplett/libgit2,joshtriplett/libgit2,Corillian/libgit2,KTXSoftware/libgit2,magnus98/TEST,magnus98/TEST,stewid/libgit2,stewid/libgit2,Corillian/libgit2,saurabhsuniljain/libgit2,Tousiph/Demo1,saurabhsuniljain/libgit2,jeffhostetler/public_libgit2,Tousiph/Demo1,Tousiph/Demo1,joshtriplett/libgit2,joshtriplett/libgit2,jeffhostetler/public_libgit2