NaCl: Fix off-by-one in getcwd() syscall emulation.

sysdeps/nacl/sysdep.h

 #ifndef _NACL_SYSDEP_H
 #define _NACL_SYSDEP_H 1
 
 #if !defined(__ASSEMBLER__)
 #include <futex_emulation.h>
 #include <irt_syscalls.h>
 #include <lowlevellock.h>
+#include <string.h>
 
 /* Implementation of all syscalls for use in platform- and OS- independent code
    as inline functions.  Each function translates the syscall arguments into IRT
    arguments and allows to intercept each call in user code.
    TODO(khim): implement the interception logic.  */
 
 # define INTERNAL_SYSCALL(name, err, nr, args...) \
   INTERNAL_SYSCALL_ ## name ## _ ## nr (&err , ## args)
 
 # define INTERNAL_SYSCALL_NCS(name, err, nr, args...) \
@@ skipping 573 matching lines @@
    The system call just returns the length of the buffer filled (which includes
    the ending '\0' character), or zero in case of error.  */
 __extern_always_inline int
 INTERNAL_SYSCALL_getcwd_2 (int *err, char *buf, size_t size)
 {
   int len;
   *err = __nacl_irt_getcwd (buf, size);
   if (*err) {
     return 0;
   }
-  for (len = 0; len < size && buf[len] != '\0'; ++len);
-  return len;
+  return __strnlen(buf, size - 1) + 1;
 }
 
 __extern_always_inline gid_t
 INTERNAL_SYSCALL_getegid_0 (int *err)
 {
   *err = (38 /* ENOSYS */);
   return 0;
 }
 
 __extern_always_inline uid_t
@@ skipping 1642 matching lines @@
 #define PSEUDO_END_ERRVAL(name)                 \
   END (name)
 
 #undef SYSCALL_ERROR_HANDLER_TLS_STORE
 #define SYSCALL_ERROR_HANDLER_TLS_STORE(src, destoff)   \
   movl %gs:0, %eax;                                     \
   movl src, (%eax,destoff)
 
 #endif
 #endif