Fix OSS-Fuzz #428983568 and #428760800

Both these issues have the same root cause, their reproducer is
extremely similar so I don't duplicate the test.

If the parser invokes the lexer, and the lexer fails, it could've
allocated a string which must be freed when the parser backs up.
The `%destructor` list is responsible for this but did not have an entry
for `fallback` yet. Solve the issue by adding such an entry.

Author: nielsdos

Zend/tests/zend_ini/oss_fuzz_428983568.phpt

@@ -0,0 +1,14 @@
+--TEST--
+OSS-Fuzz #428983568
+--FILE--
+<?php
+$ini = <<<INI
+[\${zz:-x
+
+INI;
+var_dump(parse_ini_string($ini));
+?>
+--EXPECTF--
+Warning: syntax error, unexpected end of file, expecting '}' in Unknown on line 1
+ in %s on line %d
+bool(false)

Zend/zend_ini_parser.y

@@ -353,7 +353,7 @@ static void normalize_value(zval *zv)
 %left '|' '&' '^'
 %precedence '~' '!'
 
-%destructor { zval_ini_dtor(&$$); } TC_RAW TC_CONSTANT TC_NUMBER TC_STRING TC_WHITESPACE TC_LABEL TC_OFFSET TC_VARNAME BOOL_TRUE BOOL_FALSE NULL_NULL cfg_var_ref constant_literal constant_string encapsed_list expr option_offset section_string_or_value string_or_value var_string_list var_string_list_section
+%destructor { zval_ini_dtor(&$$); } TC_RAW TC_CONSTANT TC_NUMBER TC_STRING TC_WHITESPACE TC_LABEL TC_OFFSET TC_VARNAME BOOL_TRUE BOOL_FALSE NULL_NULL cfg_var_ref constant_literal constant_string encapsed_list expr option_offset fallback section_string_or_value string_or_value var_string_list var_string_list_section
 
 %%