Harden exec_byte_code against redefining 'error'

Harden exec_byte_code against redefining 'error'

Problem discovered by configuring with --enable-gcc-warnings on
Ubuntu 17.10 x86-64 with gcc (Ubuntu 7.2.0-8ubuntu3).
* src/bytecode.c (exec_byte_code): Call the C error function
instead of the Lisp one, so that the Emacs interpreter does not go
haywire if the user redefines the Lisp error function.