Curl/Fsockopen: add some extra defensive coding

The `error_get_last()` function can return `null` if no error occurred, in which case, the `throw new Exception()` statement will run into two new errors:
* `Trying to access array offset on value of type null` for accessing `$error['message']`.
* ... which then leads to a `Exception::__construct(): Passing null to parameter #1 ($message) of type string is deprecated`.

This commit adds some defensive coding to handle the hypothetical situation that `error_get_last()` would return `null` when a file could not be opened.

Note: this is actually a bug in PHPUnit 10 which breaks `error_get_last()`.
We should be able to remove the extra defensive coding once the upstream bug has been fixed.

Upstream bug report: sebastianbergmann/phpunit#5428

src/Transport/Curl.php

@@ -177,6 +177,11 @@ public function request($url, $headers = [], $data = [], $options = []) {
 			$this->stream_handle = @fopen($options['filename'], 'wb');
 			if ($this->stream_handle === false) {
 				$error = error_get_last();
+				if (!is_array($error)) {
+					// Shouldn't be possible, but can happen in test situations.
+					$error = ['message' => 'Failed to open stream'];
+				}
+
 				throw new Exception($error['message'], 'fopen');
 			}
 		}

src/Transport/Fsockopen.php

@@ -282,6 +282,11 @@ public function request($url, $headers = [], $data = [], $options = []) {
 			$download = @fopen($options['filename'], 'wb');
 			if ($download === false) {
 				$error = error_get_last();
+				if (!is_array($error)) {
+					// Shouldn't be possible, but can happen in test situations.
+					$error = ['message' => 'Failed to open stream'];
+				}
+
 				throw new Exception($error['message'], 'fopen');
 			}
 		}