call: Properly unwind when failure

[obiwac]

This would cause problems because we'd set flamingo->env to the function's env, but then return without ever setting it back. This meant that if we failed and destroyed the flamingo object:

• First, all values are dereferenced and destroyed, which means the function's env would be destroyed.
• Then, we'd try to destroy flamingo->env, which would fail because it is one of the function environments we've already freed.
• The previous/inital flamingo->env would never be freed.

Unwind properly now and be a little less dense.

Still, Flamingo error handling needs some TLC.