asan: unwind-ia64.c: stack buffer overflow

Printing "invalid" is better than printing an uninitialised buffer
and occasionally running off the end of the buffer.

	* unwind-ia64.c (unw_print_xyreg): Don't leave output buffer
	uninitialised on invalid input.

binutils/ChangeLog

@@ -1,3 +1,8 @@
+2021-02-07  Alan Modra  <amodra@gmail.com>
+
+	* unwind-ia64.c (unw_print_xyreg): Don't leave output buffer
+	uninitialised on invalid input.
+
 2021-02-06  Alan Modra  <amodra@gmail.com>
 
 	PR 27349

binutils/unwind-ia64.c

@@ -156,6 +156,10 @@ unw_print_xyreg (char *cp, unsigned int x, unsigned int ytreg)
     case 2: /* br */
       sprintf (cp, "b%u", (ytreg & 0x1f));
       break;
+
+    default:
+      strcpy (cp, "invalid");
+      break;
     }
 }