Fix segmentation fault in banout_vprintf

Author: gpotter2

src/proto-banout.c

@@ -297,8 +297,11 @@ banout_vprintf(struct BannerOutput *banout, unsigned proto,
                const char *fmt, va_list marker) {
     char str[10];
     int len;
+    va_list marker_cpy;  // a va_list is consumed when passed to vsnprintf.
 
-    len = vsnprintf(str, sizeof(str), fmt, marker);
+    va_copy(marker_cpy, marker);
+    len = vsnprintf(str, sizeof(str), fmt, marker_cpy);
+    va_end(marker_cpy);
     if (len > sizeof(str)-1) {
         char *tmp = malloc(len+1);
         vsnprintf(tmp, len+1, fmt, marker);