DOTCODE: fix not validating ECI in every segment

Previously, only symbol->eci was checked against the 811799 upper
bound. Invalid ECI on non-first segments went unchecked, so codeword
a = (eci - 40) / 12769 + 40 could exceed 113, causing an out-of-bounds
read of dc_dot_patterns[113] in dc_make_dotstream().

backend/dotcode.c

@@ -1214,9 +1214,11 @@ INTERNAL int zint_dotcode(struct zint_symbol *symbol, struct zint_seg segs[], co
     char *dot_array;
     unsigned char *masked_codeword_array;
 
-    if (symbol->eci > 811799) {
+    for (i = 0; i < seg_count; i++) {
+        if (segs[i].eci > 811799) {
             return z_errtxtf(ZINT_ERROR_INVALID_OPTION, symbol, 525, "ECI code '%d' out of range (0 to 811799)",
-                        symbol->eci);
+                            segs[i].eci);
+        }
     }
 
     if (symbol->option_2 > 0) {