read: fix stripping of final IFS delimiters with SHOPT_MUlTIBYTE=0

The previous commit has a regression test failure when
SHOPT_MULTIBYTE is turned off in SHOPT.sh:

	builtins.sh[1775]: FAIL: issue 926 r7 (expected $'end:
	<one\ntwo\nthXree\nX>', got $'end: <one\ntwo\nthXree\n>')

read.c lines 543-568 contains a perforamnce optimisation that is
only used when SHOPT_MULTIBYTE is turned off at compile time. This
code had a bug where *all* final $IFS delimiters were stripped from
a field though only whitespace delimiters should be stripped. This
commit fixes that bug by adding an isspace(3) check for that.

Resolves: #926 (again)

Author: McDutchie

src/cmd/ksh93/bltins/read.c

@@ -553,7 +553,7 @@ int sh_readline(char **names, volatile int fd, int flags, ssize_t size, Sflong_t
 				cpmax--;
 			if(cpmax>cp)
 			{
-				while((c=sh.ifstable[*--cpmax])==S_DELIM || c==S_SPACE);
+				while((c=sh.ifstable[*--cpmax])==S_DELIM && isspace(*cpmax) || c==S_SPACE);
 				cpmax[1] = 0;
 			}
 			else