Fix nullptr access in true_regnum

[Oleg Endo]

Hi,

While working on some SH builtins I ran into the issue that the function
true_regnum in jump.c tries to access the reg_renumber array without
checking for nullptr.  The particular use case here is using true_regnum
in a predicate that is invoked during builtin function expansion in the
TARGET_EXPAND_BUILTIN function.  It seems that at that time reg_renumber
hasn't been initialized yet.  Adding the nullptr check fixes the issue.

OK for trunk?

Cheers,
Oleg

gcc/ChangeLog:
        * jump.c (true_regnum): Check that reg_renumber is not null
        before accessing it.

Index: gcc/jump.c
===================================================================
--- gcc/jump.c	(revision 216350)
+++ gcc/jump.c	(working copy)
@@ -1908,7 +1908,8 @@
   if (REG_P (x))
     {
       if (REGNO (x) >= FIRST_PSEUDO_REGISTER
-	  && (lra_in_progress || reg_renumber[REGNO (x)] >= 0))
+	  && (lra_in_progress ||
+	      (reg_renumber != NULL && reg_renumber[REGNO (x)] >= 0)))
 	return reg_renumber[REGNO (x)];
       return REGNO (x);
     }