buffer: Add and use ssh_buffer_allocate_size()

Add a small helper for ssh_buffer to ensure that the buffer has a
certain amount of space already preallocated. This can be useful in case
it is known how much data is going to be added to a buffer, to avoid
multiple reallocations.

Make use of it in few places in the library.

Signed-off-by: Pino Toscano <ptoscano@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/buffer.c

@@ -248,6 +248,34 @@ int ssh_buffer_add_data(struct ssh_buffer_struct *buffer, const void *data, uint
   return 0;
 }
 
+/**
+ * @brief Ensure the buffer has at least a certain preallocated size.
+ *
+ * @param[in]  buffer   The buffer to enlarge.
+ *
+ * @param[in]  len      The length to ensure as allocated.
+ *
+ * @return              0 on success, < 0 on error.
+ */
+int ssh_buffer_allocate_size(struct ssh_buffer_struct *buffer,
+                             uint32_t len)
+{
+    buffer_verify(buffer);
+
+    if (buffer->allocated < len) {
+        if (buffer->pos > 0) {
+            buffer_shift(buffer);
+        }
+        if (realloc_buffer(buffer, len) < 0) {
+            return -1;
+        }
+    }
+
+    buffer_verify(buffer);
+
+    return 0;
+}
+
 /**
  * @internal
  *