compiler: do not drop the loop condition at chunk offset 0

[dangowrt]

A counting for loop without an initializer that opens a function body never re-evaluates its condition. uc_compiler_compile_for_count() records where the condition starts in cond_off and treats 0 as "no condition", but when the loop is the first code in a function its condition does start at chunk offset 0, so the jump from the incrementer back to the condition is never emitted and the loop runs on after the condition has turned false:

function f(n) { for (; n > 0; n--) print(n); }
f(3);

prints 3, 2, 1, 0, -1, ... and never returns. Any statement before the loop, even let x;, moves the condition off offset 0 and hides the bug, which is why the existing for-loop test does not catch it.

The fix tests for the conditional jump instead, which is always emitted after at least one instruction of the condition and so is never at offset 0. The new regression test in 99_bugs fails with "runaway" on the old compiler instead of hanging, and the whole custom test suite passes.

[jow-]

Merged, thanks!