Fixed compiler warning.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@7612 a5019735-40e9-0310-863c-91ae7b9d1cf9
diff --git a/exp-drd/drd_intercepts.c b/exp-drd/drd_intercepts.c
index ef65455..e41d51d 100644
--- a/exp-drd/drd_intercepts.c
+++ b/exp-drd/drd_intercepts.c
@@ -171,7 +171,7 @@
 #if defined(_CS_GNU_LIBPTHREAD_VERSION)
    /* Linux with a recent glibc. */
    char buffer[256];
-   int len;
+   unsigned len;
    len = confstr(_CS_GNU_LIBPTHREAD_VERSION, buffer, sizeof(buffer));
    assert(len <= sizeof(buffer));
    return len > 0 && buffer[0] == 'l';