Use g_ascii_isspace() instead of isascii() && isspace().

This eliminates one more use of <ctype.h> macros; we're using
g_ascii_isXXX() instead of isXXX() elsewhere.

Change-Id: I184caeefab95dda2cf207acf9539eabbd7c32a51
Reviewed-on: https://code.wireshark.org/review/4767
Reviewed-by: Guy Harris <guy@alum.mit.edu>