Add a missing requires_max_content_len

Slightly reduce the amount of data so that the test passes with 512.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index d7db8ab..64e6cad 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -6616,11 +6616,12 @@
             -s "Read from client: 100 bytes read$"
 
 requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_2
+requires_max_content_len 474
 run_test    "mbedtls_ssl_get_bytes_avail: extra data" \
             "$P_SRV" \
-            "$P_CLI request_size=500" \
+            "$P_CLI request_size=450" \
             0 \
-            -s "Read from client: 500 bytes read (.*+.*)"
+            -s "Read from client: 450 bytes read (.*+.*)"
 
 # Tests for small client packets