Fix panic when dependency cycle includes the first module visited
Fix panic when dependency cycle includes the first module visited

The cycle check can panic if the first call to check happens to land
on the first module in a cycle.  Print the cycle instead of panicking.

Change-Id: I6fc1c66dcc37b1eb6b11b9e65343452af3c8538d

danw/blueprint,google/blueprint,Tiger66639/blueprint,google/blueprint,colincross/blueprint,jgennis/blueprint,colincross/blueprint,jgennis/blueprint,yuhangwang/blueprint,Tiger66639/blueprint,danw/blueprint,yuhangwang/blueprint