mesa/util: add missing va_end() after va_copy()

MSDN:
"va_end must be called on each argument list that's initialized
 with va_start or va_copy before the function returns."

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107810
Fixes: c6267ebd6c "gallium/util: Stop bundling our snprintf implementation."
Signed-off-by: Andrii Simiklit <andrii.simiklit@globallogic.com>

src/util/u_string.h

@@ -81,6 +81,7 @@ util_vsnprintf(char *str, size_t size, const char *format, va_list ap)
    if (ret < 0) {
       ret = _vscprintf(format, ap_copy);
    }
+   va_end(ap_copy);
    return ret;
 }