Test assumptions we make about the platform

Things that are not guaranteed by the standard but should be true of all
platforms of interest to us:
- 8-bit chars
- NULL pointers represented by all-bits-zero

tests/suites/main_test.function

@@ -216,6 +216,7 @@ int main()
     FILE *file;
     char buf[5000];
     char *params[50];
+    void *pointer;
 
 #if defined(MBEDTLS_MEMORY_BUFFER_ALLOC_C) && \
     !defined(TEST_SUITE_MEMORY_BUFFER_ALLOC)
@@ -223,6 +224,18 @@ int main()
     mbedtls_memory_buffer_alloc_init( alloc_buf, sizeof(alloc_buf) );
 #endif
 
+    /*
+     * The C standard doesn't guarantee that all-bits-0 is the representation
+     * of a NULL pointer. We do however use that in our code for initializing
+     * structures, which should work on every modern platform. Let's be sure.
+     */
+    memset( &pointer, 0, sizeof( void * ) );
+    if( pointer != NULL )
+    {
+        mbedtls_fprintf( stderr, "all-bits-zero is not a NULL pointer\n" );
+        return( 1 );
+    }
+
     file = fopen( filename, "r" );
     if( file == NULL )
     {