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).
diff --git a/library/cmac.c b/library/cmac.c
index 9dbff90..a76313e 100644
--- a/library/cmac.c
+++ b/library/cmac.c
@@ -832,6 +832,7 @@
         mbedtls_cipher_free( &ctx );
     }
 
+    ret = 0;
     goto exit;
 
 cleanup:
@@ -887,6 +888,7 @@
         if( verbose != 0 )
             mbedtls_printf( "passed\n" );
     }
+    ret = 0;
 
 exit:
     return( ret );