[libxml2] Update `xmlStrlen()` to use POSIX / ISO C `strlen()`

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 48ed5a74bdf24c630b99b063fa79b1a79c2ae5f3
Author: Mike Dalessio <mike dalessio gmail com>
Date:   Mon Feb 21 09:35:59 2022 -0500

    Update `xmlStrlen()` to use POSIX / ISO C `strlen()`
    
    This should be faster on a wide range of platforms.
    
    Closes #212

 xmlstring.c | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)
---
diff --git a/xmlstring.c b/xmlstring.c
index a85685d7..46301c3b 100644
--- a/xmlstring.c
+++ b/xmlstring.c
@@ -424,13 +424,7 @@ xmlStrsub(const xmlChar *str, int start, int len) {
 
 int
 xmlStrlen(const xmlChar *str) {
-    size_t len = 0;
-
-    if (str == NULL) return(0);
-    while (*str != 0) { /* non input consuming */
-        str++;
-        len++;
-    }
+    size_t len = str ? strlen((const char *)str) : 0;
     return(len > INT_MAX ? 0 : len);
 }