FENNEL:  add asserts for negative string length
FENNEL:  add asserts for negative string length

[git-p4: depot-paths = "//open/lu/dev/": change = 5287]

LucidDB/luciddb,LucidDB/luciddb,LucidDB/luciddb,LucidDB/luciddb,LucidDB/luciddb,LucidDB/luciddb,LucidDB/luciddb,LucidDB/luciddb,LucidDB/luciddb