libxl_internal.c: Fix return handling for case of mmap failure

mmap() returns MAP_FAILED not NULL.

Signed-off-by: Don Slutz <dslutz@verizon.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

tools/libxl/libxl_internal.c

@@ -246,7 +246,7 @@ int libxl__file_reference_map(libxl__file_reference *f)
 
     ret = -1;
     data = mmap(NULL, st_buf.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
-    if (data == NULL)
+    if (data == MAP_FAILED)
         goto out;
 
     f->mapped = 1;