auth_module.c: fix ssize_t check

MeeGo Coverity issue CID#7594

libsofia-sip-ua/iptsec/auth_module.c

@@ -957,12 +957,12 @@ int auth_readdb_if_needed(auth_mod_t *am)
 #define auth_apw_local ((void *)(intptr_t)auth_readdb_internal)
 
 /** Read authentication database */
-static
-int auth_readdb_internal(auth_mod_t *am, int always)
+static int auth_readdb_internal(auth_mod_t *am, int always)
 {
   FILE *f;
   char *data, *s;
-  size_t len, i, n, N;
+  ssize_t len;
+  size_t i, n, N;
   auth_passwd_t *apw;
 
   if (!am->am_stat)
@@ -1018,7 +1018,7 @@ int auth_readdb_internal(auth_mod_t *am, int always)
 
     fclose(f);
 
-    if (len < 0)
+    if (len == -1)
       return -1;
 
     /* Count number of entries in new buffer */