Minor robustness fix

src/Composer/Repository/Vcs/GitDriver.php

@@ -165,10 +165,11 @@ class GitDriver extends VcsDriver
             $this->process->execute('git branch --no-color --no-abbrev -v', $output, $this->repoDir);
             foreach ($this->process->splitLines($output) as $branch) {
                 if ($branch && !preg_match('{^ *[^/]+/HEAD }', $branch)) {
-                    preg_match('{^(?:\* )? *(?:[^/ ]+?/)?(\S+) *([a-f0-9]+) .*$}', $branch, $match);
+                    if (preg_match('{^(?:\* )? *(?:[^/ ]+?/)?(\S+) *([a-f0-9]+) .*$}', $branch, $match)) {
                        $branches[$match[1]] = $match[2];
                     }
                 }
+            }
 
             $this->branches = $branches;
         }