bug symfony#59046 [HttpClient] Fix Undefined array key connection (PhilETaylor)

This PR was submitted for the 5.4 branch but it was merged into the 6.4 branch instead.

Discussion
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[HttpClient] Fix Undefined array key `connection`

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | yes
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Issues        | Fix symfony#59039 <!-- prefix each issue number with "Fix #", no need to create an issue if none exists, explain below instead -->
| License       | MIT

Check for array key before attempting to access it
Code style for human reading

Commits
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51c32ef [HttpClient] Fix Undefined array key "connection" symfony#59044

Author: nicolas-grekas

src/Symfony/Component/HttpClient/Response/CurlResponse.php

@@ -312,7 +312,16 @@ private static function perform(ClientState $multi, ?array &$responses = null):
                 }
 
                 $multi->handlesActivity[$id][] = null;
-                $multi->handlesActivity[$id][] = \in_array($result, [\CURLE_OK, \CURLE_TOO_MANY_REDIRECTS], true) || '_0' === $waitFor || curl_getinfo($ch, \CURLINFO_SIZE_DOWNLOAD) === curl_getinfo($ch, \CURLINFO_CONTENT_LENGTH_DOWNLOAD) || (curl_error($ch) === 'OpenSSL SSL_read: SSL_ERROR_SYSCALL, errno 0' && -1.0 === curl_getinfo($ch, \CURLINFO_CONTENT_LENGTH_DOWNLOAD) && \in_array('close', array_map('strtolower', $responses[$id]->headers['connection']), true)) ? null : new TransportException(ucfirst(curl_error($ch) ?: curl_strerror($result)).sprintf(' for "%s".', curl_getinfo($ch, \CURLINFO_EFFECTIVE_URL)));
+                $multi->handlesActivity[$id][] = \in_array($result, [\CURLE_OK, \CURLE_TOO_MANY_REDIRECTS], true)
+                    || '_0' === $waitFor
+                    || curl_getinfo($ch, \CURLINFO_SIZE_DOWNLOAD) === curl_getinfo($ch, \CURLINFO_CONTENT_LENGTH_DOWNLOAD)
+                    || ('C' === $waitFor[0]
+                        && 'OpenSSL SSL_read: SSL_ERROR_SYSCALL, errno 0' === curl_error($ch)
+                        && -1.0 === curl_getinfo($ch, \CURLINFO_CONTENT_LENGTH_DOWNLOAD)
+                        && \in_array('close', array_map('strtolower', $responses[$id]->headers['connection'] ?? []), true)
+                    )
+                    ? null
+                    : new TransportException(ucfirst(curl_error($ch) ?: curl_strerror($result)).\sprintf(' for "%s".', curl_getinfo($ch, \CURLINFO_EFFECTIVE_URL)));
             }
         } finally {
             $multi->performing = false;