entropy_nv_seed: clean up properly

Call mbedtls_entropy_free on test failure.

Restore the previous NV seed functions which the call to
mbedtls_platform_set_nv_seed() changed. This didn't break anything,
but only because the NV seed functions used for these tests happened
to work for the tests that got executed later in the .data file.
diff --git a/tests/suites/test_suite_entropy.function b/tests/suites/test_suite_entropy.function
index 0b1cfe8..35efc78 100644
--- a/tests/suites/test_suite_entropy.function
+++ b/tests/suites/test_suite_entropy.function
@@ -306,6 +306,10 @@
 {
     mbedtls_sha512_context accumulator;
     mbedtls_entropy_context ctx;
+    int (*original_mbedtls_nv_seed_read)( unsigned char *buf, size_t buf_len ) =
+        mbedtls_nv_seed_read;
+    int (*original_mbedtls_nv_seed_write)( unsigned char *buf, size_t buf_len ) =
+        mbedtls_nv_seed_write;
 
     unsigned char header[2];
     unsigned char entropy[MBEDTLS_ENTROPY_BLOCK_SIZE];
@@ -372,7 +376,10 @@
     TEST_ASSERT( memcmp( check_seed, buffer_seed, MBEDTLS_ENTROPY_BLOCK_SIZE ) == 0 );
     TEST_ASSERT( memcmp( check_entropy, entropy, MBEDTLS_ENTROPY_BLOCK_SIZE ) == 0 );
 
+exit:
     mbedtls_entropy_free( &ctx );
+    mbedtls_nv_seed_read = original_mbedtls_nv_seed_read;
+    mbedtls_nv_seed_write = original_mbedtls_nv_seed_write;
 }
 /* END_CASE */