stdio: memory leak in multibyte printf

[Simon Chopin]

[Impact]
There's a memory leak when using printf for multibyte characters.

[Test case]

> test-printf.c cat <<EOF
#include <stdio.h>

int main()
{
    int i;
    for (i = 0; i < (1 << 16); ++i) printf("%.1371337ls", L"A\n");
    return 0;
}
EOF
gcc -o test-printf test-printf.c
/usr/bin/time -f '%M' ./test-printf > /dev/null

With the fix I get a 1544k footprint, without it 263232k.

[Regression potential]
The stdio subsystem is a fairly complex one and the patch isn't trivial, so we could introduce new bugs, especially in non-ASCII environments.