Check for NULL pointers in string.c

(cherry picked from commit 4230509e80)

src/string.c

@@ -92,7 +92,13 @@ int ssh_string_fill(struct ssh_string_struct *s, const void *data, size_t len) {
  */
 struct ssh_string_struct *ssh_string_from_char(const char *what) {
   struct ssh_string_struct *ptr = NULL;
-  size_t len = strlen(what);
+  size_t len;
+  
+  if(what == NULL) {
+      return NULL;
+  }
+
+  len = strlen(what);
 
   ptr = malloc(4 + len);
   if (ptr == NULL) {
@@ -133,7 +139,7 @@ size_t ssh_string_len(struct ssh_string_struct *s) {
 char *ssh_string_to_char(struct ssh_string_struct *s) {
 	size_t len;
 	char *new;
-	if(s==NULL)
+	if(s==NULL || s->string == NULL)
 		return NULL;
   len = ntohl(s->size) + 1;
   new = malloc(len);
@@ -164,7 +170,12 @@ void ssh_string_free_char(char *s) {
  * @return              Newly allocated copy of the string, NULL on error.
  */
 struct ssh_string_struct *ssh_string_copy(struct ssh_string_struct *s) {
-  struct ssh_string_struct *new = malloc(ntohl(s->size) + 4);
+  struct ssh_string_struct *new;
+  
+  if(s == NULL || s->string == NULL) {
+      return NULL;
+  }
+  new = malloc(ntohl(s->size) + 4);
 
   if (new == NULL) {
     return NULL;