m_cleanup: fix memory leaks

m_cleanup didn't cleanup the if_batchq and if_fastq queues, resulting in
a memory leak (reported by ASAN while fuzzing).

src/mbuf.c

diff --git a/src/mbuf.c b/src/mbuf.c
index 54ec721..93c27b5 100644
--- a/src/mbuf.c
+++ b/src/mbuf.c
@@ -48,6 +48,18 @@
         g_free(m);
         m = next;
     }
+    m = (struct mbuf *)slirp->if_batchq.qh_link;
+    while ((struct quehead *)m != &slirp->if_batchq) {
+        next = m->m_next;
+        g_free(m);
+        m = next;
+    }
+    m = (struct mbuf *)slirp->if_fastq.qh_link;
+    while ((struct quehead *)m != &slirp->if_fastq) {
+        next = m->m_next;
+        g_free(m);
+        m = next;
+    }
 }
 
 /*