pmcannotate: avoid accessing uninitialized local variables

Initialize `tbfl` and `tofl` to NULL, and check whether they are
non-NULL before calling remove(3) on them, to avoid warnings from clang
21 similar to:

    usr.sbin/pmcannotate/pmcannotate.c:746:3: error: variable 'tbfl' is uninitialized when used here [-Werror,-Wuninitialized]
      746 |                 FATAL(exec, "%s: Impossible to locate the binary file\n",
          |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      747 |                     exec);
          |                     ~~~~~
    usr.sbin/pmcannotate/pmcannotate.c:57:9: note: expanded from macro 'FATAL'
       57 |         remove(tbfl);                                                   \
          |                ^~~~
    usr.sbin/pmcannotate/pmcannotate.c:695:12: note: initialize the variable 'tbfl' to silence this warning
      695 |         char *tbfl, *tofl, *tmpdir;
          |                   ^
          |                    = NULL
    usr.sbin/pmcannotate/pmcannotate.c:746:3: error: variable 'tofl' is uninitialized when used here [-Werror,-Wuninitialized]
      746 |                 FATAL(exec, "%s: Impossible to locate the binary file\n",
          |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      747 |                     exec);
          |                     ~~~~~
    usr.sbin/pmcannotate/pmcannotate.c:58:9: note: expanded from macro 'FATAL'
       58 |         remove(tofl);                                                   \
          |                ^~~~
    usr.sbin/pmcannotate/pmcannotate.c:695:19: note: initialize the variable 'tofl' to silence this warning
      695 |         char *tbfl, *tofl, *tmpdir;
          |                          ^
          |                           = NULL

MFC after:	3 days

usr.sbin/pmcannotate/pmcannotate.c

@@ -54,8 +54,10 @@
 	if ((ptr) != NULL)						\
 		perror(ptr);						\
 	fprintf(stderr, ##x);						\
-	remove(tbfl);							\
-	remove(tofl);							\
+	if (tbfl != NULL)						\
+		remove(tbfl);						\
+	if (tofl != NULL)						\
+		remove(tofl);						\
 	exit(EXIT_FAILURE);						\
 } while (0)
 
@@ -702,6 +704,8 @@ main(int argc, char *argv[])
 	uintptr_t tmppc, ostart, oend;
 	int cget, asmsrc;
 
+	tbfl = NULL;
+	tofl = NULL;
 	exec = argv[0];
 	ofile = NULL;
 	bin = NULL;