Strengthen the ix zero min-clamping to panic.

Currently the ix cannot ever be less than zero (detected by Coverity).
Furthermore, it probably should never get less than zero, so panic if
that ever happens.

[perl #121897]

Fix for Coverity perl5 CID 28946:
at_least: At condition ix < 0, the value of ix must be at least 0.
dead_error_condition: The condition ix < 0 cannot be true.
Logically dead code (DEADCODE)
dead_error_line: Execution cannot reach this statement ix = 0;