Tcl_GetIndexFromObj doesn't like zero length strings

See Tk bug 1463233 for more details:

https://sourceforge.net/tracker/?func=detail&atid=112997&aid=1463233&group_id=12997

Basically, Tk's text widget tag option configuration
allows (naturally) the empty string as a valid
configuration value (to indicate that the tag does not
override the text widget for that item). This works
for all other configuration types, but not for indices
from string lists. The root cause of the problem is
that Tcl_GetIndexFromObj throws an error in this case,
even though there is an exact match available (and
nowhere in the documentation is it stated that an exact
match will not be found for the special case of an
empty string).

An apparent fix would be:

/*
* The key should not be empty, otherwise it's not a
match.
*/

if (key[0] == '\0') {
+
+ /*
+ * Check for special case of "" in string
table.
+ */
+ for (entryPtr = tablePtr, i = 0; *entryPtr !=
NULL;
+ entryPtr = NEXT_ENTRY(entryPtr, offset), i++) {
+ if (**entryPtr == '\0') {
+ index = i;
+ goto done;
+ }
+ }
+
goto error;
}

/*
* Scan the table looking for one of:

Note that this does NOT give the empty string any
special treatment - it simply equalises the treatment
with what happens with any other prefix string. If
there is an exact match to any prefix string it is
accepted, even if there are longer strings which could
also match.

Vince.