frontend: Check file before creating print job (#78)

Try to open the file at the given file path before
creating a print job using `cpdbPrintFD`.

Otherwise, passing an invalid path causes a print
job to be created that never gets processed.

For example, with cpdb-text-frontend, passing
an invalid path would print an error message:

    > print-file /dummy/nonexistent PDF CUPS
    [Error] [Frontend] Error opening file /dummy/nonexistent on PDF CUPS: No such file or directory

But it would still create a print job:

    $ lpstat
    PDF-209                 michi                0   2024-11-20T09:01:02 CET

That one would never get finished and has to be manually
cancelled using `cancel <jobid>`.

(I noticed this because on KDE Plamsa, the printer widget
was showing after that failed attempt, due to the pending
print job.)

With this commit in place, the same error message is
shown, but no print job is created.

cpdb/cpdb-frontend.c

@@ -1148,6 +1148,13 @@ char *cpdbPrintFile(cpdb_printer_obj_t *p,
 char *cpdbPrintFileWithJobTitle(cpdb_printer_obj_t *p,
                     const char *file_path, const char *title)
 {
+    FILE *file = fopen(file_path, "r");
+    if (file == NULL) {
+        logerror("Error opening file %s on %s %s: %s\n",
+                 file_path, p->id, p->backend_name, strerror(errno));
+        return NULL;
+    }
+
     char *jobid = NULL;
     char *socket_path = NULL;
     int fd = cpdbPrintFD(p, &jobid, title, &socket_path);
@@ -1158,15 +1165,6 @@ char *cpdbPrintFileWithJobTitle(cpdb_printer_obj_t *p,
         return NULL;
     }
 
-    FILE *file = fopen(file_path, "r");
-    if (file == NULL) {
-        close(fd);
-        logerror("Error opening file %s on %s %s: %s\n",
-                 file_path, p->id, p->backend_name, strerror(errno));
-        g_free(socket_path);
-        return NULL;
-    }
-
     char buffer[1024];
     size_t bytesRead;