Fix insert() of an empty range blanking the tail

[martinus]

Fixes #65.

v.insert(pos, 0, value) blanked every element from pos onwards. No exception, no sanitizer report, just wrong data:

insert(begin(), 0, value)
  svector       [<EMPTY!> <EMPTY!> <EMPTY!> ]
  std::vector   [aaaa... bbbb... cccc... ]

Cause

make_uninitialized_space calls shift_right(p, data + s, p + count). With count == 0 the target begin equals the source begin, so the second step becomes std::move_backward(first, last, last) — destination equal to source end, which that algorithm explicitly disallows. It walks backwards self-move-assigning every element in the range.

Affects insert(pos, 0, value), insert(pos, first, first) (forward iterators) and insert(pos, {}). The input-iterator overload already had an empty-range early-out, which is why it escaped.

Where the guard goes

In shift_right, not at the call site. Its documented precondition is source_begin <= target_begin — non-strict, so the equal case was always advertised as valid input and the implementation simply did not honour it. Guarding the one caller instead would leave the primitive a footgun for the next one.

The review raised that count == 0 at the call site is easier for the compiler to fold than pointer equality inside the callee. Benchmarked both placements:

	guard in shift_right	guard in caller
insert(begin(), value) int	19.55 / 19.77 ns	19.63 / 19.52 ns
insert(mid, value) int	13.07 / 13.04 ns	13.05 / 13.61 ns
insert(begin(), string)	138.15 / 137.58 ns	137.13 / 135.97 ns

No difference, so correctness of the primitive wins.

Tests

Two cases in test/unit/insert.cpp, covering all three affected overloads at every position including the pos == size boundary, and once more in indirect mode. Both fail without the guard.

They use std::string deliberately. This file already had empty-range insert coverage, but with Counter::Obj, whose move assignment is a plain field copy — a self-move preserves the value, so the corruption was unobservable. That blind spot is #67.

Verified with gcc and clang under -fsanitize=address,undefined.