fix: use resize instead of reserve in batch_invert to fix UB

batch_invert used reserve() for its temporaries and skipped vectors,
then accessed elements via operator[]. reserve() only allocates capacity
without changing size, making operator[] access undefined behavior.
This crashes in debug builds with _GLIBCXX_ASSERTIONS enabled.

Fixed by using direct size-construction: vector(n) instead of
reserve(n).

Author: AztecBot

barretenberg/cpp/src/barretenberg/ecc/fields/field_impl.hpp

@@ -419,10 +419,8 @@ void field<T>::batch_invert(C& coeffs) noexcept
 {
     const size_t n = coeffs.size();
 
-    std::vector<field> temporaries;
-    std::vector<bool> skipped;
-    temporaries.reserve(n);
-    skipped.reserve(n);
+    std::vector<field> temporaries(n);
+    std::vector<bool> skipped(n);
 
     field accumulator = one();
     for (size_t i = 0; i < n; ++i) {