Ensure free space in Vector is zero-initialized

[pgaskin]

This fixes a lack of reproducibility when writing the FlatVector LoudsTrie.extras_, which comes from the Vector<uint32_t> terminals, which is set to the Tail offsets during Tail::build from LoudsTrie::build_next_trie.

[pgaskin]

Building words.txt before and after this change:

diff --git a/before b/after
index 2b19266..6705ebd 100644
--- a/before
+++ b/after
@@ -82100,7 +82100,7 @@
 00140b30: 010a 0101 0101 0101 2401 6701 0100 0805  ........$.g.....
 00140b40: 014a 0101 0101 0101 0101 0100 002b 0100  .J...........+..
 00140b50: 0000 1909 094a 094a 092b 1000 4a4a 0900  .....J.J.+..JJ..
-00140b60: 01fc 0100 0000 0000 0800 0000 ff00 0000  ................
+00140b60: 0100 0000 0000 0000 0800 0000 ff00 0000  ................
 00140b70: 990b 0500 0000 0000 0000 0000 0000 0000  ................
 00140b80: 0000 0000 0000 0000 0000 0000 0000 0000  ................
 00140b90: 0000 0000 0000 0000 0000 0000 0000 0000  ................

Stack trace of the write which wrote that offset:

marisa.write(i32,i32)
marisa.wasm.marisa::grimoire::io::Writer::data(void const*, unsigned long)(i32,i32,i32)
marisa.wasm.void marisa::grimoire::io::Writer::write<unsigned int>(unsigned int const*, unsigned long)(i32,i32,i32)
marisa.wasm.marisa::grimoire::vector::Vector<unsigned int>::write(marisa::grimoire::io::Writer&) const(i32,i32)
marisa.wasm.marisa::grimoire::vector::FlatVector::write(marisa::grimoire::io::Writer&) const(i32,i32)
marisa.wasm.marisa::grimoire::trie::LoudsTrie::write_(marisa::grimoire::io::Writer&) const(i32,i32)
marisa.wasm.marisa::grimoire::trie::LoudsTrie::write(marisa::grimoire::io::Writer&) const(i32,i32)
marisa.wasm.marisa::Trie::write(int) const(i32,i32)
marisa.wasm.marisa_save()

The buffer of length 330656 passed to the write function (i.e., The uint32_t* Vector<uint32_t>.const_objs_ of FlatVector.units_ of LoudsTrie.extra_):

--- snip ---
00050b90  09 2b 10 00 4a 4a 09 00  01 fc 01 00 00 00 00 00  |.+..JJ..........|

[pgaskin]

Note that the output of a simple marisa-build command isn't changed by this PR, as it's uninitialized memory happens to be zero.

This is an alternative patch which would have the same effect:

diff --git a/lib/marisa/grimoire/vector/vector.h b/lib/marisa/grimoire/vector/vector.h
index afc7d9a..b25031c 100644
--- a/lib/marisa/grimoire/vector/vector.h
+++ b/lib/marisa/grimoire/vector/vector.h
@@ -256,7 +256,7 @@ class Vector {
     assert(new_capacity >= size_);
     assert(new_capacity <= max_size());
 
-    std::unique_ptr<char[]> new_buf(new char[sizeof(T) * new_capacity]);
+    std::unique_ptr<char[]> new_buf(new char[sizeof(T) * new_capacity]());
     T *new_objs = reinterpret_cast<T *>(new_buf.get());
 
     static_assert(std::is_trivially_copyable_v<T>);
@@ -273,7 +273,7 @@ class Vector {
   void copyInit(const T *src, std::size_t size, std::size_t capacity) {
     assert(size_ == 0);
 
-    std::unique_ptr<char[]> new_buf(new char[sizeof(T) * capacity]);
+    std::unique_ptr<char[]> new_buf(new char[sizeof(T) * capacity]());
     T *new_objs = reinterpret_cast<T *>(new_buf.get());
 
     static_assert(std::is_trivially_copyable_v<T>);