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;

pp_ctl.c

@@ -3098,7 +3098,7 @@ PP(pp_goto) /* also pp_dump */
 	    I32 oldsave;
 
 	    if (ix < 0)
-		ix = 0;
+		DIE(aTHX_ "panic: docatch: illegal ix=%ld", (long)ix);
 	    dounwind(ix);
 	    TOPBLOCK(cx);
 	    oldsave = PL_scopestack[PL_scopestack_ix];