MINOR: cache: replace a fprint() by an abort()

In the applet I/O handler we can never get an object bigger than a
buffer, so we should never reach this case.
diff --git a/src/cache.c b/src/cache.c
index 5af51f2..3447038 100644
--- a/src/cache.c
+++ b/src/cache.c
@@ -560,7 +560,9 @@
 	if (appctx->st0 == HTTP_CACHE_INIT) {
 		int len = first->len - sizeof(struct cache_entry);
 		if ((shctx_row_data_get(shctx, first, (unsigned char *)bi_end(res->buf), sizeof(struct cache_entry), len)) != 0) {
-			fprintf(stderr, "cache error too big: %d\n", first->len - (int)sizeof(struct cache_entry));
+			/* should never get there, because at the moment, a
+			 * cache object can never be bigger than a buffer */
+			 abort();
 
 			si_applet_cant_put(si);
 			goto out;