Do not pass NULL for the string in catch_errors

I caught a segmentation fault while running gdb.reverse/sigall-reverse.exp,
in a mingw32 GDB, in this code path. It boils down to the code trying to
strlen () a NULL pointer. I tracked things down and it looks like
record_full_message_wrapper_safe is the only offender.

gdb/ChangeLog:

2015-10-26  Luis Machado  <lgustavo@codesourcery.com>

	* record-full.c (record_full_message_wrapper_safe): Pass empty string to
	catch_errors call instead of NULL.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2015-10-26  Luis Machado  <lgustavo@codesourcery.com>
+
+	* record-full.c (record_full_message_wrapper_safe): Pass empty string to
+	catch_errors call instead of NULL.
+
 2015-10-26  Simon Marchi  <simon.marchi@polymtl.ca>
 
 	* guile/scm-ports.c (ioscm_make_gdb_stdio_port): Pass non-const

gdb/record-full.c

@@ -667,7 +667,7 @@ record_full_message_wrapper_safe (struct regcache *regcache,
   args.regcache = regcache;
   args.signal = signal;
 
-  return catch_errors (record_full_message_wrapper, &args, NULL,
+  return catch_errors (record_full_message_wrapper, &args, "",
 		       RETURN_MASK_ALL);
 }