civetweb.c: check for array boundry before read

Added assert to while loop to make sure not to read over the array boundry.

Fix for Coverity issue from Ceph project:

CID 1160832 (#1 of 1): Out-of-bounds read (OVERRUN_STATIC)
 37. overrun-local: Overrunning static array "buf", with 8192
 elements, at position 8192 with index variable "len".

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>

src/civetweb.c

@@ -5325,6 +5325,7 @@ int mg_upload(struct mg_connection *conn, const char *destination_dir)
         assert(len >= 0 && len <= (int) sizeof(buf));
         while ((n = mg_read(conn, buf + len, sizeof(buf) - len)) > 0) {
             len += n;
+            assert(len <= (int) sizeof(buf));
         }
         if ((headers_len = get_request_len(buf, len)) <= 0) {
             break;