linux-user: in poll(), if nfds is 0, pfd can be NULL

This problem appears with yum in Fedora 20 / PPC64 container.

test case:

    #include <stdio.h>
    #include <poll.h>

    int main(void)
    {
        int ret;

        ret = poll(NULL, 0, 1000);
        printf("%d\n", ret);
    }

target test environment: Fedora 20 / PPC64
host test environment: Ubuntu 14.0.2 / x86_64

original test result: -1

    13451 poll(0,0,1000,274886297496,268566664,268566648) = -1 errno=14 (Bad address)

patched test result: 0

    13536 poll(0,0,1000,274886297496,268566664,268566648) = 0
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>
Signed-off-by: NRiku Voipio <riku.voipio@linaro.org>

linux-user/syscall.c

@@ -8046,15 +8046,21 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
             struct pollfd *pfd;
             unsigned int i;
 
-            target_pfd = lock_user(VERIFY_WRITE, arg1, sizeof(struct target_pollfd) * nfds, 1);
-            if (!target_pfd)
+            pfd = NULL;
+            target_pfd = NULL;
+            if (nfds) {
+                target_pfd = lock_user(VERIFY_WRITE, arg1,
+                                       sizeof(struct target_pollfd) * nfds, 1);
+                if (!target_pfd) {
                     goto efault;
+                }
 
                 pfd = alloca(sizeof(struct pollfd) * nfds);
-            for(i = 0; i < nfds; i++) {
+                for (i = 0; i < nfds; i++) {
                     pfd[i].fd = tswap32(target_pfd[i].fd);
                     pfd[i].events = tswap16(target_pfd[i].events);
                 }
+            }
 
 # ifdef TARGET_NR_ppoll
             if (num == TARGET_NR_ppoll) {