Generic/InlineControlStructure: fix unchecked findNext() return value

On line 67, `findNext()` searches for the first non-empty token after
a `T_ELSE` to detect the `else if` pattern. When `else` is the last
non-empty token in the file (live coding/parse error), `findNext()`
returns `false`. This `false` value was used directly as an array
index on line 68 (`$tokens[$next]`), which silently becomes
`$tokens[0]`, reading the `T_OPEN_TAG` token instead. The bug was silent
because the code would eventually bail out at line 81-83.

This commit adds a `$next === false` check, which also allows the
sniff to bail out earlier for an `else` at end of file instead of
continuing to do unnecessary work before reaching the existing live
coding bail-out further down.

Existing test file InlineControlStructureUnitTest.12.inc already
covers this scenario.

Author: rodrigoprimo

src/Standards/Generic/Sniffs/ControlStructures/InlineControlStructureSniff.php

@@ -65,7 +65,7 @@ public function process(File $phpcsFile, int $stackPtr)
         // Ignore the ELSE in ELSE IF. We'll process the IF part later.
         if ($tokens[$stackPtr]['code'] === T_ELSE) {
             $next = $phpcsFile->findNext(Tokens::EMPTY_TOKENS, ($stackPtr + 1), null, true);
-            if ($tokens[$next]['code'] === T_IF) {
+            if ($next === false || $tokens[$next]['code'] === T_IF) {
                 return;
             }
         }