[gdb/breakpoints] Don't ignore <file> in rbreak <file>:<regexp>

PR breakpoints/34112 reports that "rbreak <file>:<regexp>" sets breakpoints in
files other than <file>.

This is a regression since commit c4c093a31f6 ("Make
global_symbol_searcher::filenames private"), which did:
...
   if (file_name != nullptr)
-    spec.filenames.push_back (file_name);
+    spec.add_filename (std::move (file_name));
...

The std::move nullifies file_name, so a subsequent file_name check:
...
	  if (file_name != nullptr)
...
now always evaluates to false.

Fix this by:
- introducing a variable bool file_name_p, initialized before the
  std::move, and
- using that instead.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=34112