eth/downloader: fix rollback issue on short chains

[holiman]

In 8cbdc86#diff-c2fa15e758e986688c646459d8970a50R1501-R1504 , the rollback mechanism was changed, from beign a slice of hashes to being a number.
The change also cause a regression, if the number is smaller than fsHeaderSafetyNet, then the rollback will be 0, and the sethead won't happen. This causes some spurious test failures,

[user@work go-ethereum]$ go test ./eth/downloader/ -run InvalidHeaderRollback 
--- FAIL: TestInvalidHeaderRollback63Fast (2.39s)
    downloader_test.go:1047: rollback head mismatch: have 384, want at most 192
--- FAIL: TestInvalidHeaderRollback65Light (3.23s)
    downloader_test.go:1047: rollback head mismatch: have 960, want at most 192
FAIL
FAIL    github.com/ethereum/go-ethereum/eth/downloader  6.513s

[karalabe]

LGTM