pem_read_buffer() already update use_len after header and footer are read

After header and footer are read, pem_read_buffer() is able to determine
the length of input data used. This allows calling functions to skip
this PEM bit if an error occurs during its parsing.
diff --git a/include/polarssl/error.h b/include/polarssl/error.h
index 093bec1..73ebe89 100644
--- a/include/polarssl/error.h
+++ b/include/polarssl/error.h
@@ -73,7 +73,7 @@
  *
  * High-level module nr (3 bits - 0x1...-0x8...)
  * Name     ID  Nr of Errors
- * PEM      1   8
+ * PEM      1   9
  * X509     2   21
  * DHM      3   6
  * RSA      4   9