fix leak in ranap_new_transp_layer_addr()

Like BIT_STRING_fromBuf(), free any buffer previously kept in the target
BIT_STRING_t.

Context: osmo-hnbgw calls ranap_new_transp_layer_addr() to replace an
address in a decoded message, before re-encoding it. So far doing so
leaked the previous buffer.

Change-Id: Ic92fa317644c3a1f2be9badf455d34fe0b8ea2a4
---
M src/iu_helpers.c
1 file changed, 18 insertions(+), 0 deletions(-)

diff --git a/src/iu_helpers.c b/src/iu_helpers.c
index 6e2c203..718c30a 100644
--- a/src/iu_helpers.c
+++ b/src/iu_helpers.c
@@ -217,6 +217,8 @@
 		buf = CALLOC(len, sizeof(uint8_t));
 		memcpy(buf, ip_addr, ip_len);
 	}
+	if (out->buf)
+		FREEMEM(out->buf);
 	out->buf = buf;
 	out->size = len;
 	out->bits_unused = 0;

