XUtils: add safe strncpy implementation

The standard strncpy fails to null-terminate the destination in case
the source is longer than the passed size.

XUtils.c

@@ -193,6 +193,18 @@ char* String_readLine(FILE* fd) {
    }
 }
 
+size_t String_safeStrncpy(char *restrict dest, const char *restrict src, size_t size) {
+   assert(size > 0);
+
+   size_t i = 0;
+   for (; i < size - 1 && src[i]; i++)
+      dest[i] = src[i];
+
+   dest[i] = '\0';
+
+   return i;
+}
+
 int xAsprintf(char** strp, const char* fmt, ...) {
    va_list vl;
    va_start(vl, fmt);