Fix varargs misuse in sbuffer module

lib/sbuffer.c

@@ -45,7 +45,7 @@ struct string_buffer_t
 #define GROW(s,l)                                                       \
     do {                                                                \
         (s)->size = ((((s)->len + (l)) / (s)->block) + 1) * (s)->block; \
-        (s)->buffer = xrealloc((s)->buffer, (s)->len);                  \
+        (s)->buffer = xrealloc((s)->buffer, (s)->size);                 \
     } while ( 0 )
 
 #else
@@ -192,20 +192,19 @@ int
 sb_vaddf(string_buffer_t *s, const char *fmt, va_list ap)
 {
     int written;
+    va_list aq;
 
     if ( ! s || ! fmt ) {
         errno = EINVAL;
         return -1;
     }
 
-    written = vsnprintf(CURSOR(s), AVAILABLE_SIZE(s), fmt, ap);
+    va_copy(aq, ap);
+    written = vsnprintf(CURSOR(s), AVAILABLE_SIZE(s), fmt, aq);
+    va_end(aq);
     if ( written >= AVAILABLE_SIZE(s) ) {
-        va_list aq;
-
         GROW(s, written);
-        va_copy(aq, ap);
-        vsprintf(CURSOR(s), fmt, aq);
-        va_end(aq);
+        vsprintf(CURSOR(s), fmt, ap);
     }
     s->len += written;