scan-gram: avoid portability trap with ctype usage.

* src/scan-gram.l (<SC_ESCAPED_STRING,SC_ESCAPED_CHARACTER>):
Avoid compiler warning.

Signed-off-by: Eric Blake <ebb9@byu.net>
(cherry picked from commit bbbbe221d7)

src/scan-gram.l

@@ -618,7 +618,7 @@ splice	 (\\[ \f\t\v]*\n)*
   \\(.|\n)	{
     char const *p = yytext + 1;
     /* Quote only if escaping won't make the character visible.  */
-    if (isspace (*p) && isprint (*p))
+    if (isspace ((unsigned char) *p) && isprint ((unsigned char) *p))
       p = quote (p);
     else
       p = quotearg_style_mem (escape_quoting_style, p, 1);