slirp: Cleanup and basic reanimation of debug code

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/slirp/bootp.c b/slirp/bootp.c
index 97c2811..fec52c0 100644
--- a/slirp/bootp.c
+++ b/slirp/bootp.c
@@ -42,7 +42,7 @@
 
 #ifdef DEBUG
 #define dprintf(fmt, ...) \
-if (slirp_debug & DBG_CALL) { fprintf(dfd, fmt, ##  __VA_ARGS__); fflush(dfd); }
+do if (slirp_debug & DBG_CALL) { fprintf(dfd, fmt, ##  __VA_ARGS__); fflush(dfd); } while (0)
 #else
 #define dprintf(fmt, ...)
 #endif