core: fix pointer access in msgb_l[1-4] macros

Put the 'm' pointer into braces, so that it's possible to pass an
expression to these macros, e.g. a pointer-to-pointer dereference.

This patch makes the following example compile:

  struct msgb **msg = /* ... */;
  return msgb_l2(*msg); /* <-- currently this fails */

Currently it fails with the following error:

 error: ‘msg’ is a pointer to pointer; did you mean to
        dereference it before applying ‘->’ to it?

Change-Id: I2d19ea3c09ff9499314255d408fb71c07148fe25
---
M include/osmocom/core/msgb.h
1 file changed, 26 insertions(+), 4 deletions(-)

diff --git a/include/osmocom/core/msgb.h b/include/osmocom/core/msgb.h
index 2529c0e..5c58c84 100644
--- a/include/osmocom/core/msgb.h
+++ b/include/osmocom/core/msgb.h
@@ -127,13 +127,13 @@
 #endif
 
 /*! obtain L1 header of msgb */
-#define msgb_l1(m)	((void *)(m->l1h))
+#define msgb_l1(m)	((void *)((m)->l1h))
 /*! obtain L2 header of msgb */
-#define msgb_l2(m)	((void *)(m->l2h))
+#define msgb_l2(m)	((void *)((m)->l2h))
 /*! obtain L3 header of msgb */
-#define msgb_l3(m)	((void *)(m->l3h))
+#define msgb_l3(m)	((void *)((m)->l3h))
 /*! obtain L4 header of msgb */
-#define msgb_l4(m)	((void *)(m->l4h))
+#define msgb_l4(m)	((void *)((m)->l4h))
 /*! obtain SMS header of msgb */
 #define msgb_sms(m)	msgb_l4(m)
 

