Fix compilation errors.

Under gcc11(+) both message and received would cause errors for
potentially being used uninitialised. We fixed many of these issues in
another PR, but this one is only seen under certain configs.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>

ChangeLog.d/fix_compilation_ssl_tests.txt

@@ -0,0 +1,3 @@
+Bugfix
+   * Fix an uninitialized variable  warning in test_suite_ssl.function with GCC
+     version 11

tests/suites/test_suite_ssl.function

@@ -2236,8 +2236,8 @@ exit:
 void ssl_mock_sanity( )
 {
     enum { MSGLEN = 105 };
-    unsigned char message[MSGLEN];
-    unsigned char received[MSGLEN];
+    unsigned char message[MSGLEN] = { 0 };
+    unsigned char received[MSGLEN] = { 0 };
     mbedtls_mock_socket socket;
 
     mbedtls_mock_socket_init( &socket );