files: limit the number of attempts at climbing up the directory tree

Doing a chdir("..") will not fail when the root directory is reached,
and when getcwd() keeps failing too, we have no way of knowing when
to stop.  So, simply limit the number of attempted chdirs, to avoid
getting into an endless loop.

This avoids the hang in https://savannah.gnu.org/bugs/index.php?47659

.
Reported-by: Chris Renshaw <osm0sis@outlook.com>
Signed-off-by: Benno Schulenberg <bensberg@justemail.net>

src/files.c

@@ -1409,6 +1409,8 @@ void do_insertfile_void(void)
  * able to go there. */
 char *get_full_path(const char *origpath)
 {
+    int attempts = 0;
+	/* How often we've tried climing back up the tree. */
     struct stat fileinfo;
     char *d_here, *d_there, *d_there_file = NULL;
     const char *last_slash;
@@ -1422,11 +1424,10 @@ char *get_full_path(const char *origpath)
      * current directory. */
     d_here = getcwd(NULL, PATH_MAX + 1);
 
-    while (d_here == NULL) {
-	if (chdir("..") == -1)
-	    break;
-
+    while (d_here == NULL && attempts < 20) {
+	IGNORE_CALL_RESULT(chdir(".."));
 	d_here = getcwd(NULL, PATH_MAX + 1);
+	attempts++;
     }
 
     /* If we succeeded, canonicalize it in d_here. */