Fix phpGH-17399: iconv memory leak on bailout

Wrap bailable sections in php_iconv_string(), _php_iconv_substr(),
_php_iconv_mime_encode(), and _php_iconv_mime_decode() with zend_try/zend_catch
to ensure iconv handles allocated via system malloc are closed if a Zend OOM
bailout fires during smart_str or zend_string operations.

Fixes phpGH-17399
Closes phpGH-21541

Author: iliaal

NEWS

@@ -6,6 +6,9 @@ PHP                                                                        NEWS
   . Fixed bug GH-19983 (GC assertion failure with fibers, generators and
     destructors). (iliaal)
 
+- Iconv:
+  . Fixed bug GH-17399 (iconv memory leak on bailout). (iliaal)
+
 - SPL:
   . Fixed bug GH-21499 (RecursiveArrayIterator getChildren UAF after parent
     free). (Girgias)