Mp4: prevent chunk index underflow.

When cropping stsc atom, it's assumed that chunk index is never 0.
Based on this assumption, start_chunk and end_chunk are calculated
by subtracting 1 from it.  If chunk index is zero, start_chunk or
end_chunk may underflow, which will later trigger
"start/end time is out mp4 stco chunks" error.  The change adds an
explicit check for zero chunk index to avoid underflow and report
a proper error.

Zero chunk index is explicitly banned in ISO/IEC 14496-12, 8.7.4
Sample To Chunk Box.  It's also implicitly banned in QuickTime File
Format specification.  Description of chunk offset table references
"Chunk 1" as the first table element.

src/http/modules/ngx_http_mp4_module.c

@@ -3221,6 +3221,12 @@ found:
         return NGX_ERROR;
     }
 
+    if (chunk == 0) {
+        ngx_log_error(NGX_LOG_ERR, mp4->file.log, 0,
+                      "zero chunk in \"%s\"", mp4->file.name.data);
+        return NGX_ERROR;
+    }
+
     target_chunk = chunk - 1;
     target_chunk += start_sample / samples;
     chunk_samples = start_sample % samples;