Fix segfault in learn-c.org/xx/Linked lists.md

Account for edge case in remove_by_index when index = size of the array.

Author: memst

tutorials/learn-c.org/de/Linked lists.md

@@ -210,6 +210,10 @@ There are a few edge cases we need to take care of, so make sure you understand
             current = current->next;
         }
 
+        if (current->next == NULL) {
+            return -1;
+        }
+
         temp_node = current->next;
         retval = temp_node->val;
         current->next = temp_node->next;

tutorials/learn-c.org/en/Linked lists.md

@@ -223,6 +223,10 @@ There are a few edge cases we need to take care of, so make sure you understand
             current = current->next;
         }
 
+        if (current->next == NULL) {
+            return -1;
+        }
+
         temp_node = current->next;
         retval = temp_node->val;
         current->next = temp_node->next;

tutorials/learn-c.org/nl/Linked lists.md

@@ -223,6 +223,10 @@ There are a few edge cases we need to take care of, so make sure you understand
             current = current->next;
         }
 
+        if (current->next == NULL) {
+            return -1;
+        }
+
         temp_node = current->next;
         retval = temp_node->val;
         current->next = temp_node->next;