Ensure dst_vec is initialized after init_copy()

[eramongodb]

Address the following compiler warnings which are causing hardened-compile task failure:

In file included from src/common/tests/test-mlib.c:1294:
src/common/tests/test-mlib.c: In function '_test_cstring_vec':
src/common/src/mlib/vec.th:143:29: error: 'b.size' may be used uninitialized in this function [-Werror=maybe-uninitialized]
    return v->data ? v->data + v->size : v->data;
                             ^
src/common/tests/test-mlib.c:1329:19: note: 'b.size' was declared here
       cstring_vec b;
                   ^
In file included from src/common/tests/test-mlib.c:1294:
src/common/src/mlib/vec.th:241:4: error: 'b.data' may be used uninitialized in this function [-Werror=maybe-uninitialized]
    for (T *r_iter = last; r_iter != first; --r_iter) {
    ^~~
src/common/tests/test-mlib.c:1329:19: note: 'b.data' was declared here
       cstring_vec b;
                   ^

This appears to be due to the init_copy() function containing code paths which may fail to initialize *dst_vec (when either reservation or element copy fails). This can leave *dst_vec with indeterminate values which are subsequently accessed by test assertions.

This PR proposes always leaving *dst_vec in a (zero-)initialized state even on failure to address these warnings and potential UB.

[kevinAlbs]

Thank you for the fix. LGTM!