bug #838 [Flex] Fix Composer update exception when flex://defaults missing (gnito-org)

This PR was squashed before being merged into the 1.x branch.

Discussion
----------

[Flex] Fix Composer update exception when flex://defaults missing

Q | A
-- | --
Branch? | 1.x
Bug fix? | yes
New feature? | no
Deprecations? | no
Tickets | Fix #836
License | MIT
Doc PR | -

Commits
-------

767b8c0 [Flex] Fix Composer update exception when flex://defaults missing

Author: nicolas-grekas

src/Cache.php

@@ -130,6 +130,9 @@ private function getVersions(): array
         $this->downloader = null;
         $okVersions = [];
 
+        if (!isset($versions['splits'])) {
+            throw new \LogicException('The Flex index is missing a "splits" entry. Did you forget to add "flex://defaults" in the "extra.symfony.endpoint" array of your composer.json?');
+        }
         foreach ($versions['splits'] as $name => $vers) {
             foreach ($vers as $i => $v) {
                 if (!isset($okVersions[$v])) {

src/PackageFilter.php

@@ -122,6 +122,9 @@ private function getVersions(): array
         $this->downloader = null;
         $okVersions = [];
 
+        if (!isset($versions['splits'])) {
+            throw new \LogicException('The Flex index is missing a "splits" entry. Did you forget to add "flex://defaults" in the "extra.symfony.endpoint" array of your composer.json?');
+        }
         foreach ($versions['splits'] as $name => $vers) {
             foreach ($vers as $i => $v) {
                 if (!isset($okVersions[$v])) {