Avoid passing NULL to strdup()

Cherry-picked from
ali-rantakari/peg-markdown-highlight@6d501919b30ca18980030df8

Fix #532.

Author: ali-rantakari

Dependency/peg-markdown-highlight/pmh_grammar.leg

@@ -483,8 +483,8 @@ ReferenceLinkDouble =  < s:Label Spnl !"[]" l:Label >
                         	pmh_realelement *reference = GET_REF(l->label);
                             if (reference) {
                                 $$ = elem_s(pmh_LINK);
-                                $$->label = strdup(l->label);
-                                $$->address = strdup(reference->address);
+                                $$->label = strdup_or_null(l->label);
+                                $$->address = strdup_or_null(reference->address);
                             } else
                                 $$ = NULL;
                             FREE_LABEL(s);
@@ -496,8 +496,8 @@ ReferenceLinkSingle =  < s:Label (Spnl "[]")? >
                         	pmh_realelement *reference = GET_REF(s->label);
                             if (reference) {
                                 $$ = elem_s(pmh_LINK);
-                                $$->label = strdup(s->label);
-                                $$->address = strdup(reference->address);
+                                $$->label = strdup_or_null(s->label);
+                                $$->address = strdup_or_null(reference->address);
                             } else
                                 $$ = NULL;
                             FREE_LABEL(s);
@@ -507,7 +507,7 @@ ExplicitLink =  < s:Label Spnl '(' Sp l:Source Spnl Title Sp ')' >
                 {
                     $$ = elem_s(pmh_LINK);
                     if (l->address != NULL)
-                        $$->address = strdup(l->address);
+                        $$->address = strdup_or_null(l->address);
                     FREE_LABEL(s);
                     FREE_ADDRESS(l);
                 }
@@ -550,8 +550,8 @@ Reference = < s:LocMarker
               NonindentSpace !"[]" l:Label ':' Spnl r:RefSrc RefTitle > BlankLine+
               {
                 pmh_realelement *el = elem_s(pmh_REFERENCE);
-                el->label = strdup(l->label);
-                el->address = strdup(r->address);
+                el->label = strdup_or_null(l->label);
+                el->address = strdup_or_null(r->address);
                 ADD(el);
                 FREE_LABEL(l);
                 FREE_ADDRESS(r);

Dependency/peg-markdown-highlight/pmh_parser_head.c

@@ -25,6 +25,11 @@
 #endif
 
 
+char *strdup_or_null(char *s)
+{
+    return (s == NULL) ? NULL : strdup(s);
+}
+
 
 // Alias strdup to _strdup on MSVC:
 #ifdef _MSC_VER
@@ -704,9 +709,9 @@ static pmh_realelement *mk_element(parser_data *p_data, pmh_element_type type,
 static pmh_realelement *copy_element(parser_data *p_data, pmh_realelement *elem)
 {
     pmh_realelement *result = mk_element(p_data, elem->type, elem->pos, elem->end);
-    result->label = (elem->label == NULL) ? NULL : strdup(elem->label);
-    result->text = (elem->text == NULL) ? NULL : strdup(elem->text);
-    result->address = (elem->address == NULL) ? NULL : strdup(elem->address);
+    result->label = strdup_or_null(elem->label);
+    result->text = strdup_or_null(elem->text);
+    result->address = strdup_or_null(elem->address);
     return result;
 }
 
@@ -716,7 +721,7 @@ static pmh_realelement *mk_etext(parser_data *p_data, char *string)
     pmh_realelement *result;
     assert(string != NULL);
     result = mk_element(p_data, pmh_EXTRA_TEXT, 0,0);
-    result->text = strdup(string);
+    result->text = strdup_or_null(string);
     return result;
 }