slirp: remove dead nested assignment, spotted by clang

Although the value stored to 'r' is used in the enclosing expression,
the value is never actually read from 'r'.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

slirp/slirp.c

diff --git a/slirp/slirp.c b/slirp/slirp.c
index 6731f52..3c785cd 100644
--- a/slirp/slirp.c
+++ b/slirp/slirp.c
@@ -1069,9 +1069,8 @@
 {
     Slirp *slirp = opaque;
     struct ex_list *ex_ptr;
-    int r;
 
-    while ((r = qemu_get_byte(f))) {
+    while (qemu_get_byte(f)) {
         int ret;
         struct socket *so = socreate(slirp);