Call setbuf when reading or writing files: programs

After opening a file containing sensitive data, call mbedtls_setbuf() to
disable buffering. This way, we don't expose sensitive data to a memory
disclosure vulnerability in a buffer outside our control.

This commit adds a call to mbedtls_setbuf() after each call to fopen(),
but only in sample programs that were calling mbedtls_platform_zeroize().
Don't bother protecting stdio buffers in programs where application buffers
weren't protected.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

programs/ssl/ssl_test_lib.h

@@ -35,6 +35,7 @@
 #define mbedtls_fprintf    fprintf
 #define mbedtls_snprintf   snprintf
 #define mbedtls_exit            exit
+#define mbedtls_setbuf     setbuf
 #define MBEDTLS_EXIT_SUCCESS    EXIT_SUCCESS
 #define MBEDTLS_EXIT_FAILURE    EXIT_FAILURE
 #endif