Fix #77141: Signedness issue in SOAP when precision=-1

According to php_gcvt(), we assume at most 17 fractional digits for
negative precision.

NEWS

@@ -11,6 +11,7 @@ PHP                                                                        NEWS
 
 - SOAP:
   . Fixed bug #76348 (WSDL_CACHE_MEMORY causes Segmentation fault). (cmb)
+  . Fixed bug #77141 (Signedness issue in SOAP when precision=-1). (cmb)
 
 08 Nov 2018, PHP 7.1.24
 

ext/soap/php_encoding.c

@@ -1098,7 +1098,7 @@ static xmlNodePtr to_xml_double(encodeTypePtr type, zval *data, int style, xmlNo
 
 	ZVAL_DOUBLE(&tmp, zval_get_double(data));
 
-	str = (char *) safe_emalloc(EG(precision), 1, MAX_LENGTH_OF_DOUBLE + 1);
+	str = (char *) safe_emalloc(EG(precision) >= 0 ? EG(precision) : 17, 1, MAX_LENGTH_OF_DOUBLE + 1);
 	php_gcvt(Z_DVAL(tmp), EG(precision), '.', 'E', str);
 	xmlNodeSetContentLen(ret, BAD_CAST(str), strlen(str));
 	efree(str);

ext/soap/tests/bugs/bug77141.phpt

@@ -0,0 +1,27 @@
+--TEST--
+Bug #77141 (Signedness issue in SOAP when precision=-1)
+--SKIPIF--
+<?php
+if (!extension_loaded('soap')) die('skip soap extension not available');
+?>
+--FILE--
+<?php
+$soap = new \SoapClient(
+    null,
+    array(
+        'location' => "http://localhost/soap.php",
+        'uri' => "http://localhost/",
+        'style' => SOAP_RPC,
+        'trace' => true,
+        'exceptions' => false,
+    )
+);
+ini_set('precision', -1);
+$soap->call(1.1);
+echo $soap->__getLastRequest();
+?>
+===DONE===
+--EXPECT--
+<?xml version="1.0" encoding="UTF-8"?>
+<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ns1="http://localhost/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:SOAP-ENC="http://schemas.xmlsoap.org/soap/encoding/" SOAP-ENV:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"><SOAP-ENV:Body><ns1:call><param0 xsi:type="xsd:float">1.1</param0></ns1:call></SOAP-ENV:Body></SOAP-ENV:Envelope>
+===DONE===