Avoid invalid read and uninitialized comparisons
https://bugs.gentoo.org/915782
https://puszcza.gnu.org.ua/bugs/?611

This backports the following upstream commits:

r1387
Author: karl
Date: Thu Oct 12 16:10:29 2023 UTC
avoid negative index into font_tbl

r1391
Author: karl
Date: Fri Oct 13 22:34:24 2023 UTC
zero htf_4hf array to avoid uninitialized reads

--- tex4ht-1.0.2009_06_11_1038/src/tex4ht.c
+++ tex4ht-1.0.2009_06_11_1038/src/tex4ht.c
@@ -7039,6 +7039,7 @@
 
 max_htf_4hf_n = 256;
 htf_4hf = m_alloc(struct htf_4hf_rec, 256);
+memset (htf_4hf, 0, 256 * sizeof (struct htf_4hf_rec));
 
 
       
@@ -8738,7 +8739,7 @@
 
    
 if( span_on && !in_span_ch  && !ignore_chs && !in_accenting
-            && (default_font != font_tbl[cur_fnt].num) ){
+            && cur_fnt >= 0 && (default_font != font_tbl[cur_fnt].num) ){
   if(  (ch < 137) && (ch != 
 132 
 ) ){