Update library/ssl_msg.c

Co-authored-by: Gilles Peskine <gilles.peskine@arm.com>
Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/ssl_msg.c b/library/ssl_msg.c
index 1162cca..5deaeda 100644
--- a/library/ssl_msg.c
+++ b/library/ssl_msg.c
@@ -5593,7 +5593,7 @@
     n = (len < ssl->in_msglen)
         ? len : ssl->in_msglen;
 
-    if (buf) {
+    if (len != 0) {
         memcpy(buf, ssl->in_offt, n);
         ssl->in_msglen -= n;
     }