Validate output length in the child

If the child reports an output length that's larger than the buffer, report
it rather than overread the buffer. The parent would catch the excess
length anyway, but having the child catch it makes the behavior more uniform
with respect to the presence or absence of sanitizers.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

Author: gilles-peskine-arm

tests/src/fork_helpers.c

@@ -80,6 +80,8 @@ static void run_child(
 
     child_callback(param, buf, size, &length);
 
+    TEST_LE_U(length, size);
+
     if (mbedtls_test_get_result() == MBEDTLS_TEST_RESULT_SUCCESS && length != 0) {
         /* Write the output. This could fail on a full disk. Remember to
          * flush (otherwise the output would likely be truncated). */