Fix incorrect buf size argument caught by Coverity-scan

src/sbbs3/qwk.cpp

@@ -585,7 +585,7 @@ void sbbs_t::qwk_sec()
 			off_t l=flength(str);
 			bprintf(text[FiFilename], getfname(str));
 			bprintf(text[FiFileSize], ultoac((ulong)l,tmp)
-				, byte_estimate_to_str(l, tmp2, sizeof(tmp), /* units: */1024, /* precision: */1));
+				, byte_estimate_to_str(l, tmp2, sizeof(tmp2), /* units: */1024, /* precision: */1));
 
 			if(l>0L && cur_cps)
 				i=(uint)(l/(ulong)cur_cps);