gzlib: Add check for res of LSEEK

Return value of function LSEEK (lseek64) is not checked, but it is
usually checked for this function.

Found by RASU JSC.

gzlib.c

@@ -244,7 +244,8 @@ local gzFile gz_open(const void *path, int fd, const char *mode) {
         return NULL;
     }
     if (state->mode == GZ_APPEND) {
-        LSEEK(state->fd, 0, SEEK_END);  /* so gzoffset() is correct */
+        if (LSEEK(state->fd, 0, SEEK_END) == -1)  /* so gzoffset() is correct */
+            return NULL;
         state->mode = GZ_WRITE;         /* simplify later checks */
     }