ext/sodium: general buffer upper bounds adjustments.

For zend_string allocations, it was tested against SIZE_MAX now using
ZSTR_MAX_LEN to take in account zend_string type's metadata.

close GH-17588

Author: devnexen

NEWS

@@ -95,6 +95,10 @@ PHP                                                                        NEWS
   . Fixed bug #49169 (SoapServer calls wrong function, although "SOAP action"
     header is correct). (nielsdos)
 
+- Sodium:
+  . Fix overall theorical overflows on zend_string buffer allocations.
+    (David Carlier/nielsdos)
+
 - Sockets:
   . Added IPPROTO_ICMP/IPPROTO_ICMPV6 to create raw socket for ICMP usage.
     (David Carlier)