Fix a memory leak in x509write test suite

This leak wasn't discovered by the CI because the only test in
all.sh exercising the respective path enabled the custom memory
buffer allocator implementations of calloc() and free(), hence
bypassing ASan.

tests/suites/test_suite_x509write.function

@@ -57,6 +57,7 @@ static int x509_crt_verifycsr( const unsigned char *buf, size_t buflen )
         return( MBEDTLS_ERR_X509_CERT_VERIFY_FAILED );
     }
 
+    mbedtls_x509_csr_free( &csr );
     return( 0 );
 }
 #endif /* MBEDTLS_USE_PSA_CRYPTO */