kvp string: allocate enough space

We need to allocate enough space for the terminating null character.
Also, I double-checked the documents for std::basic_string::c_str () and
verified that it does guarantee the terminating null, so I put a comment
in the code that depends on that.

libgnucash/engine/gnc-budget.c

@@ -489,7 +489,7 @@ void
 gnc_budget_unset_account_period_value(GncBudget *budget, const Account *account,
                                       guint period_num)
 {
-    gchar path_part_one [GUID_ENCODING_LENGTH];
+    gchar path_part_one [GUID_ENCODING_LENGTH + 1];
     gchar path_part_two [GNC_BUDGET_MAX_NUM_PERIODS_DIGITS];
 
     g_return_if_fail (budget != NULL);
@@ -511,7 +511,7 @@ void
 gnc_budget_set_account_period_value(GncBudget *budget, const Account *account,
                                     guint period_num, gnc_numeric val)
 {
-    gchar path_part_one [GUID_ENCODING_LENGTH];
+    gchar path_part_one [GUID_ENCODING_LENGTH + 1];
     gchar path_part_two [GNC_BUDGET_MAX_NUM_PERIODS_DIGITS];
 
     /* Watch out for an off-by-one error here:
@@ -553,7 +553,7 @@ gnc_budget_is_account_period_value_set(const GncBudget *budget,
                                        guint period_num)
 {
     GValue v = G_VALUE_INIT;
-    gchar path_part_one [GUID_ENCODING_LENGTH];
+    gchar path_part_one [GUID_ENCODING_LENGTH + 1];
     gchar path_part_two [GNC_BUDGET_MAX_NUM_PERIODS_DIGITS];
     gconstpointer ptr = NULL;
 
@@ -573,7 +573,7 @@ gnc_budget_get_account_period_value(const GncBudget *budget,
                                     guint period_num)
 {
     gnc_numeric *numeric = NULL;
-    gchar path_part_one [GUID_ENCODING_LENGTH];
+    gchar path_part_one [GUID_ENCODING_LENGTH + 1];
     gchar path_part_two [GNC_BUDGET_MAX_NUM_PERIODS_DIGITS];
     GValue v = G_VALUE_INIT;
 

libgnucash/engine/guid.cpp

@@ -177,7 +177,9 @@ guid_to_string_buff (const GncGUID * guid, gchar *str)
 
     gnc::GUID temp {*guid};
     auto val = temp.to_string ();
-    /*We need to be sure to copy the terminating null character.*/
+    /*We need to be sure to copy the terminating null character.
+     * The standard guarantees that std::basic_string::c_str ()
+     * returns with a terminating null character, too.*/
     std::copy (val.c_str (), val.c_str () + val.size () + 1, str);
     return str + val.size ();
 }