check return value of unw_get_reg and unw_get_proc_name

Before printng the current procedure name and IP verify their validity
diff --git a/proc.h b/proc.h
index 64f8fe2..35943c3 100644
--- a/proc.h
+++ b/proc.h
@@ -30,6 +30,7 @@
 
 #if defined(HAVE_LIBUNWIND)
 # include <libunwind.h>
+# include <libunwind-ptrace.h>
 #endif /* defined(HAVE_LIBUNWIND) */
 
 #include "ltrace.h"