treat null vdso base same as missing

On s390x, the kernel provides AT_SYSINFO_EHDR, but sets it to zero, if the
program being run does not have a program interpreter.  This causes
problems when running the dynamic linker directly.

ldso/dynlink.c

@@ -1550,7 +1550,7 @@ _Noreturn void __dls3(size_t *sp)
 	}
 
 	/* Attach to vdso, if provided by the kernel */
-	if (search_vec(auxv, &vdso_base, AT_SYSINFO_EHDR)) {
+	if (search_vec(auxv, &vdso_base, AT_SYSINFO_EHDR) && vdso_base) {
 		Ehdr *ehdr = (void *)vdso_base;
 		Phdr *phdr = vdso.phdr = (void *)(vdso_base + ehdr->e_phoff);
 		vdso.phnum = ehdr->e_phnum;

src/internal/vdso.c

@@ -45,6 +45,7 @@ void *__vdsosym(const char *vername, const char *name)
 	size_t i;
 	for (i=0; libc.auxv[i] != AT_SYSINFO_EHDR; i+=2)
 		if (!libc.auxv[i]) return 0;
+	if (!libc.auxv[i+1]) return 0;
 	Ehdr *eh = (void *)libc.auxv[i+1];
 	Phdr *ph = (void *)((char *)eh + eh->e_phoff);
 	size_t *dynv=0, base=-1;