Fix bug in gzgets() for a concatenated empty gzip stream.

gzread.c

@@ -569,15 +569,15 @@ char * ZEXPORT gzgets(file, buf, len)
     left = (unsigned)len - 1;
     if (left) do {
         /* assure that something is in the output buffer */
-        if (state->have == 0) {
+        while (state->have == 0 && (state->strm.avail_in || !state->eof)) {
             if (gz_make(state) == -1)
                 return NULL;            /* error */
+        }
         if (state->have == 0) {         /* end of file */
             if (buf == str)             /* got bupkus */
                 return NULL;
             break;                      /* got something -- return it */
         }
-        }
 
         /* look for end-of-line in current output buffer */
         n = state->have > left ? left : state->have;