vector: add convenience access methods

Accessing the last element of a vector is a common operation.
Having it named and bound checked would definetely be a plus for
`vlc_vector` usage. It avoids error-prones index accesses.

include/vlc_vector.h

@@ -664,6 +664,24 @@ vlc_vector_move_(char *array, size_t index, size_t count, size_t target)
              ((item) = (pv)->data[vlc_vector_idx_##item], true); \
          ++vlc_vector_idx_##item)
 
+/**
+ * Returns a reference to the vector's first element.
+ */
+#define vlc_vector_last(pv) \
+( \
+    assert((pv)->size != 0), \
+    (pv)->data[(pv)->size - 1] \
+)
+
+/**
+ * Returns a reference on the vector's last element.
+ */
+#define vlc_vector_last_ref(pv) \
+( \
+    assert((pv)->size != 0), \
+    &(pv)->data[(pv)->size - 1] \
+)
+
 /** @} */
 
 #endif

src/test/vector.c

@@ -39,6 +39,8 @@ static void test_vector_insert_remove(void)
     assert(ok);
     assert(vec.data[0] == 42);
     assert(vec.size == 1);
+    assert(vlc_vector_last(&vec) == 42);
+    assert(*vlc_vector_last_ref(&vec) == 42);
 
     ok = vlc_vector_push(&vec, 37);
     assert(ok);
@@ -66,6 +68,8 @@ static void test_vector_insert_remove(void)
     assert(vec.data[0] == 42);
     assert(vec.data[1] == 37);
     assert(vec.data[2] == 77);
+    assert(vlc_vector_last(&vec) == 77);
+    assert(*vlc_vector_last_ref(&vec) == 77);
 
     vlc_vector_clear(&vec);
     assert(vec.size == 0);