Fix GH-15712: overflow on float print with precision large value.

When allocating enough room for floats, the allocator used by overflows with
large ndigits/EG(precision) value which used an signed integer to double
the buffer. Testing with the zend operator directly is enough to trigger
 the issue rather than higher level math interface.

Author: devnexen

Zend/tests/gh15712.phpt

@@ -0,0 +1,9 @@
+--TEST--
+GH-15712: overflow on real number printing
+--FILE--
+<?php
+ini_set('precision', 1100000000);
+echo  -1 * (2 ** -10);
+?>
+--EXPECTF--
+%s