Pacify compiler warning.

src/archiver_zip.c

@@ -1587,7 +1587,7 @@ static void ZIP_closeArchive(void *opaque);
 static void *ZIP_openArchive(PHYSFS_Io *io, const char *name, int forWriting)
 {
     ZIPinfo *info = NULL;
-    PHYSFS_uint64 dstart;  /* data start */
+    PHYSFS_uint64 dstart = 0;  /* data start */
     PHYSFS_uint64 cdir_ofs;  /* central dir offset */
     PHYSFS_uint64 entry_count;