[BUG] capture: do not capture a cookie if there is no memory left

In case of out of memory, it was possible to write to a null pointer
when capturing response cookies due to a missing "else" block. The
request handling was fine though.
(cherry picked from commit 62e3604d7dd27741c0b4c9e27d9e7c73495dfc32)
diff --git a/src/proto_http.c b/src/proto_http.c
index db86769..e06a782 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -6766,11 +6766,12 @@
 				if ((txn->srv_cookie = pool_alloc2(pool2_capture)) == NULL) {
 					Alert("HTTP logging : out of memory.\n");
 				}
-
-				if (log_len > t->fe->capture_len)
-					log_len = t->fe->capture_len;
-				memcpy(txn->srv_cookie, att_beg, log_len);
-				txn->srv_cookie[log_len] = 0;
+				else {
+					if (log_len > t->fe->capture_len)
+						log_len = t->fe->capture_len;
+					memcpy(txn->srv_cookie, att_beg, log_len);
+					txn->srv_cookie[log_len] = 0;
+				}
 			}
 
 			/* now check if we need to process it for persistence */