fix 2ndheader logging null-deref

Fixes fuzz GH LibreDWG#890

Author: rurban

src/decode.c

@@ -2611,7 +2611,8 @@ secondheader_private (Bit_Chain *restrict dat, Dwg_Data *restrict dwg)
 {
   Bit_Chain *str_dat = dat;
   Dwg_SecondHeader *_obj = &dwg->secondheader;
-  Dwg_Object *obj = NULL;
+  // for error logging only:
+  Dwg_Object *obj = &(Dwg_Object){ .name = (char*)"2NDHEADER" };
   int error = 0;
   BITCODE_BL vcount;
   if (!dat->chain || !dat->size)

src/encode.c

@@ -2353,7 +2353,8 @@ encode_secondheader_private (Bit_Chain *restrict dat, Dwg_Data *restrict dwg)
 {
   Bit_Chain *str_dat = dat;
   Dwg_SecondHeader *_obj = &dwg->secondheader;
-  Dwg_Object *obj = NULL;
+  // for error logging only:
+  Dwg_Object *obj = &(Dwg_Object){ .name = (char*)"2NDHEADER" };
   int error = 0;
   BITCODE_BL vcount;
   if (!dat->chain || !dat->size)

src/spec.h

@@ -76,15 +76,15 @@
 #    define VALUEOUTOFBOUNDS(field, maxvalue)                                 \
       if (_IN_RANGE (_obj->field, maxvalue) && _obj->field > maxvalue)        \
         {                                                                     \
-          LOG_ERROR ("Invalid %s." #field " %lu", obj->name,                  \
+          LOG_ERROR ("Invalid %s." #field " %lu", obj ? obj->name : "",       \
                      (unsigned long)_obj->field);                             \
           _obj->field = 0;                                                    \
           return DWG_ERR_VALUEOUTOFBOUNDS;                                    \
         }
 #    define SUB_VALUEOUTOFBOUNDS(o, field, maxvalue)                          \
       if (_IN_RANGE (_obj->o.field, maxvalue) && _obj->o.field > maxvalue)    \
         {                                                                     \
-          LOG_ERROR ("Invalid %s." #field " %lu", obj->name,                  \
+          LOG_ERROR ("Invalid %s." #field " %lu", obj ? obj->name : "",       \
                      (unsigned long)_obj->o.field);                           \
           _obj->o.field = 0;                                                  \
           return DWG_ERR_VALUEOUTOFBOUNDS;                                    \