Fix --logfile and message truncation issue.
Regression in 6.4.20's security fix (Git commit c546c829). We doubly incremented partial_message_size_used on modern systems (stdard.h/vsnprintf), once in report_vbuild() and then again in report_build(), so the 2nd and subsequent report_build() fragments landed too late in the buffer. This will not cause overruns due to the reallocation prior to the vsnprintf/sprintf, but it write starts behind the '\0' byte, instead of right over it, so the string also gets truncated to the first fragment written with report_vbuild(). Fix by moving the increment back into the #else...#endif part that does not use report_vbuild(). Reported by: Jürgen Edner, Erik Christiansen