is_driftnet_file: Refactor file name checking code (import from Debian)

driftnet.c

@@ -89,9 +89,7 @@ void clean_temporary_directory(void) {
         buf = xmalloc(buflen = strlen(tmpdir) + 64);
 
         while ((de = readdir(d))) {
-            char *p;
-            p = strrchr(de->d_name, '.');
-            if (!tmpdir_specified || (p && strncmp(de->d_name, "driftnet-", 9) == 0 && (strcmp(p, ".jpeg") == 0 || strcmp(p, ".gif") == 0 || strcmp(p, ".mp3") == 0))) {
+            if (!tmpdir_specified || is_driftnet_file(de->d_name)) {
                 if (buflen < strlen(tmpdir) + strlen(de->d_name) + 1)
                     buf = xrealloc(buf, buflen = strlen(tmpdir) + strlen(de->d_name) + 64);
 

driftnet.h

@@ -81,6 +81,7 @@ connection *find_connection(const struct in_addr *src, const struct in_addr *dst
 
 /* media.c */
 void connection_extract_media(connection c, const enum mediatype T);
+int is_driftnet_file(char *filename);
 
 /* util.c */
 void *xmalloc(size_t n);

media.c

@@ -39,6 +39,16 @@ void dispatch_http_req(const char *mname, const unsigned char *data, const size_
 /* playaudio.c */
 void mpeg_submit_chunk(const unsigned char *data, const size_t len);
 
+int is_driftnet_file(char *filename) {
+    if (strncmp(filename, "driftnet-", 9) != 0) return 0;
+    char *p = strrchr(filename, '.');
+    if (p == 0) return 0;
+    return (strcmp(p, ".jpeg") == 0 ||
+           strcmp(p, ".gif") == 0 ||
+           strcmp(p, ".png") == 0 ||
+           strcmp(p, ".mp3") == 0);
+}
+
 /* count_temporary_files:
  * How many of our files remain in the temporary directory? We do this a
  * maximum of once every five seconds. */