wasm: directly handle recycling of invalidated instance

An instance may be invalidated before we try to recycle it.
We perform this by setting its value to a nullopt.
This patch adds a check for it when calculating its size.
This behavior didn't cause issues before because the catch
clause below caught errors caused by calling value() on
a nullopt, even though it was intended for errors from
get_instance_size.

Signed-off-by: Wojciech Mitros <wojciech.mitros@scylladb.com>

Closes #11500

lang/wasm_instance_cache.cc

@@ -147,6 +147,9 @@ void instance_cache::recycle(instance_cache::value_type val) noexcept {
     val->mutex.unlock();
     size_t size;
     try {
+        if (!val->instance) {
+            return;
+        }
         size = get_instance_size(val->instance.value());
         if (size > 1 * MB) {
             val->instance = std::nullopt;