wrapper: guard writev_in_full() against signed overflow

As Git for Windows' Coverity run after merging v2.56.0-rc0 reported,
`writev_in_full()` keeps its cumulative successful output in an
`ssize_t`. Although `xwritev()` limits each individual write to a
syscall-sized amount, repeated successful writes can still exceed
`SSIZE_MAX`. The unchecked accumulation was introduced by d70eb7f360
(wrapper: introduce writev(3p) wrappers, 2026-08-07).

Treat an aggregate that would overflow the signed total as an I/O
failure.

Assisted-by: GPT-5.6 Luna
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

wrapper.c

@@ -376,6 +376,10 @@ ssize_t writev_in_full(int fd, struct iovec *iov, int iovcnt)
 			return -1;
 		}
 
+		if (signed_add_overflows(total_written, bytes_written)) {
+			errno = EOVERFLOW;
+			return -1;
+		}
 		total_written += bytes_written;
 
 		/*