Fix build with gcc -O -Wall

Fix warnings from gcc -O -Wall about `ret` used uninitialized in
CMAC selftest auxiliary functions. The variable was indeed
uninitialized if the function was called with num_tests=0 (which
never happens).

library/cmac.c

@@ -832,6 +832,7 @@ static int cmac_test_subkeys( int verbose,
         mbedtls_cipher_free( &ctx );
     }
 
+    ret = 0;
     goto exit;
 
 cleanup:
@@ -887,6 +888,7 @@ static int cmac_test_wth_cipher( int verbose,
         if( verbose != 0 )
             mbedtls_printf( "passed\n" );
     }
+    ret = 0;
 
 exit:
     return( ret );