dump_privatekey with FILETYPE_TEXT only supports RSA keys (#646)

* dump_privatekey with FILETYPE_TEXT only supports RSA keys

FILETYPE_TEXT is terrible but everyone hold their nose

* also verify it's a pkey
diff --git a/src/OpenSSL/crypto.py b/src/OpenSSL/crypto.py
index ef2dcdf..5803ae9 100644
--- a/src/OpenSSL/crypto.py
+++ b/src/OpenSSL/crypto.py
@@ -1837,6 +1837,9 @@
     """
     bio = _new_mem_buf()
 
+    if not isinstance(pkey, PKey):
+        raise TypeError("pkey must be a PKey")
+
     if cipher is not None:
         if passphrase is None:
             raise TypeError(
@@ -1857,6 +1860,9 @@
     elif type == FILETYPE_ASN1:
         result_code = _lib.i2d_PrivateKey_bio(bio, pkey._pkey)
     elif type == FILETYPE_TEXT:
+        if _lib.EVP_PKEY_id(pkey._pkey) != _lib.EVP_PKEY_RSA:
+            raise TypeError("Only RSA keys are supported for FILETYPE_TEXT")
+
         rsa = _ffi.gc(
             _lib.EVP_PKEY_get1_RSA(pkey._pkey),
             _lib.RSA_free