Fix xvasprintf function

The xvasprintf function made improper use of the va_* macros
to handle variable argument lists.

lib/xmem.c

@@ -123,13 +123,14 @@ xvasprintf(char **s, const char *fmt, va_list ap)
     int n;
     va_list aq;
 
-    n = vsnprintf(NULL, 0, fmt, ap) + 1;
+    va_copy(aq, ap);
+    n = vsnprintf(NULL, 0, fmt, aq) + 1;
+    va_end(aq);
+
     if ( ! (*s = malloc(n)) )
         mem_error((size_t) n);
 
-    va_copy(ap, aq);
-    n = vsnprintf(*s, n, fmt, aq);
-    va_end(aq);
+    n = vsnprintf(*s, n, fmt, ap);
 
     return n;
 }