linux-user: fix QEMU_STRACE=1 segfault

While debugging some issues with QEMU_STRACE I stumbled over segmentation
faults that were pretty reproducible. Turns out we tried to treat a
normal return value as errno, resulting in an access over array boundaries
for the resolution.

Fix this by allowing failure to resolve invalid errnos into strings.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>

linux-user/syscall.c

@@ -731,6 +731,9 @@ static inline int is_error(abi_long ret)
 
 char *target_strerror(int err)
 {
+    if ((err >= ERRNO_TABLE_SIZE) || (err < 0)) {
+        return NULL;
+    }
     return strerror(target_to_host_errno(err));
 }