HTTP/2: avoid memcpy() with NULL source and zero length.

Prodded by Clang Static Analyzer.

src/http/v2/ngx_http_v2.c

@@ -4241,7 +4241,9 @@ ngx_http_v2_process_request_body(ngx_http_request_t *r, u_char *pos,
                 n = size;
             }
 
-            rb->buf->last = ngx_cpymem(rb->buf->last, pos, n);
+            if (n > 0) {
+                rb->buf->last = ngx_cpymem(rb->buf->last, pos, n);
+            }
 
             ngx_log_debug1(NGX_LOG_DEBUG_HTTP, fc->log, 0,
                            "http2 request body recv %uz", n);