[ARM] Minor fix in arm_function_ok_for_sibcall

[Eric Botcazou]

Several tests in the function were updated with a guard for a NULL decl but 
not the VxWorks-specific one, which results in a segfault building libgcc.

Tested on ARM/VxWorks, applied on mainline, 6 and 5 branches as obvious.


2016-07-01  Eric Botcazou  <ebotca...@adacore.com>

        * config/arm/arm.c (arm_function_ok_for_sibcall): Add another check
        for NULL decl.

-- 
Eric Botcazou
Index: config/arm/arm.c
===================================================================
--- config/arm/arm.c	(revision 237902)
+++ config/arm/arm.c	(working copy)
@@ -6756,7 +6756,7 @@ arm_function_ok_for_sibcall (tree decl,
 
   /* The PIC register is live on entry to VxWorks PLT entries, so we
      must make the call before restoring the PIC register.  */
-  if (TARGET_VXWORKS_RTP && flag_pic && !targetm.binds_local_p (decl))
+  if (TARGET_VXWORKS_RTP && flag_pic && decl && !targetm.binds_local_p (decl))
     return false;
 
   /* If we are interworking and the function is not declared static