mb_ereg_search_setpos(): Add support for negative position

Also add missing test for this function

ext/mbstring/php_mbregex.c

@@ -1400,6 +1400,11 @@ PHP_FUNCTION(mb_ereg_search_setpos)
 		return;
 	}
 
+	/* Accept negative position if length of search string can be determined */
+	if ((position < 0) && (!Z_ISUNDEF(MBREX(search_str))) && (Z_TYPE(MBREX(search_str)) == IS_STRING)) {
+		position += Z_STRLEN(MBREX(search_str));
+	}
+		
 	if (position < 0 || (!Z_ISUNDEF(MBREX(search_str)) && Z_TYPE(MBREX(search_str)) == IS_STRING && (size_t)position >= Z_STRLEN(MBREX(search_str)))) {
 		php_error_docref(NULL, E_WARNING, "Position is out of range");
 		MBREX(search_pos) = 0;

ext/mbstring/tests/mb_ereg_search_setpos.phpt

@@ -0,0 +1,70 @@
+--TEST--
+mb_ereg_search_setpos() function
+--SKIPIF--
+<?php
+if (!extension_loaded('mbstring')) die('skip mbstring not enabled');
+?>
+--FILE--
+<?php
+mb_regex_encoding('iso-8859-1');
+$test_str = 'Iñtërnâtiônàlizætiøn'; // Length = 20
+
+var_dump(mb_ereg_search_setpos(50)); // OK
+var_dump(mb_ereg_search_setpos(-1)); // Error
+
+mb_ereg_search_init($test_str);
+
+$positions = array( 5, 19, 20, 25, 0, -5, -20, -30);
+foreach($positions as $pos) {
+	echo("\n* Position: $pos :\n");
+	var_dump(mb_ereg_search_setpos($pos));
+	var_dump(mb_ereg_search_getpos());
+}
+?>
+==DONE==
+--EXPECTF--
+bool(true)
+
+Warning: mb_ereg_search_setpos(): Position is out of range in %s on line %d
+bool(false)
+
+* Position: 5 :
+bool(true)
+int(5)
+
+* Position: 19 :
+bool(true)
+int(19)
+
+* Position: 20 :
+
+Warning: mb_ereg_search_setpos(): Position is out of range in %s on line %d
+bool(false)
+int(0)
+
+* Position: 25 :
+
+Warning: mb_ereg_search_setpos(): Position is out of range in %s on line %d
+bool(false)
+int(0)
+
+* Position: 0 :
+bool(true)
+int(0)
+
+* Position: -5 :
+bool(true)
+int(15)
+
+* Position: -20 :
+bool(true)
+int(0)
+
+* Position: -30 :
+
+Warning: mb_ereg_search_setpos(): Position is out of range in %s on line %d
+bool(false)
+int(0)
+==DONE==
+
+