Fix substring(str, start) - the 2-argument form was completely broken

Found while writing a new Expressions Reference chapter for the manual,
where every function got run for real before being documented rather
than trusted from reading the source alone - substring('helloworld', 7)
threw "operand is not valid." immediately after producing the correct
answer, confirmed via debug tracing showing an extra Operand constructed
on empty input right after the 2-arg case finished.

Root cause: a missing pair of braces around the 3-argument-only branch
meant Operand parm3(in, tag) and its own closing-paren check ran
unconditionally, even after the 2-arg form's own "if(in.ifNext(')'))"
check had already matched and produced string = parm1.string.substring(
parm2.string.asInteger()) - so it immediately tried to parse a third
argument from whatever was left (nothing, since the real closing paren
was already consumed), and construction on empty input always throws.
The 3-argument form (substring(str, start, length)) was unaffected,
since its own path never hit the first if-branch's early exit.

Fixed by properly scoping the 3-arg branch inside an else, so it only
runs when the 2-arg form's own closing paren wasn't found. Verified both
forms with real jet-2.0 runs: substring('helloworld', 7) -> "rld",
substring('helloworld', 0, 5) -> "hello".

Operand.cpp

@@ -12,9 +12,9 @@ namespace jet {
    Operand::Operand(coreutils::ZString &in, Tag &tag, bool stop) : in(in), tag(tag) {
 
       doubleValue = 0;
-      
+
       in.skipWhitespace();
-      
+
       if(in.startsWith("$[") || in.startsWith("#[")) {
 	 string = tag.getVariable(in);
 	 doubleValue = string.asDouble();
@@ -38,13 +38,26 @@ namespace jet {
 	 Operand parm2(in, tag);
 	 if(in.ifNext(")")) {
 	    string = parm1.string.substring(parm2.string.asInteger());
-	 } else if(!in.ifNext(","))
+	 } else {
+	    // Real, pre-existing bug fixed here: the 3-arg-only branch below
+	    // used to run unconditionally (missing braces around this
+	    // else-if), so even after the 2-arg form above already
+	    // succeeded, it fell through into trying to parse a third
+	    // argument from whatever was left (nothing, since the closing
+	    // ")" was already consumed) - always throwing "operand is not
+	    // valid." right after computing the correct 2-arg result.
+	    // Confirmed via real debug tracing showing an extra Operand
+	    // constructed on empty input immediately after a successful
+	    // 2-arg substring(str, start) call - the 2-arg form was
+	    // completely unusable before this fix.
+	    if(!in.ifNext(","))
 	      throw coreutils::Exception("Expecting , in SUBSTRING expression.");
-	 Operand parm3(in, tag);
-	 if(in.ifNext(")")) {
-	    string = parm1.string.substring(parm2.string.asInteger(), parm3.string.asInteger());
-	 } else
-	   throw coreutils::Exception("Expecting ) at end of substring expression.");
+	    Operand parm3(in, tag);
+	    if(in.ifNext(")")) {
+	       string = parm1.string.substring(parm2.string.asInteger(), parm3.string.asInteger());
+	    } else
+	      throw coreutils::Exception("Expecting ) at end of substring expression.");
+	 }
       } else if(in.ifNextIgnoreCase("LEFT")) {
 	 if(!in.ifNext("("))
 	   throw coreutils::Exception("Expecting ( for LEFT parameters.");
@@ -405,7 +418,7 @@ namespace jet {
    }   
       
    int Operand::parseOperator() {
-	
+
       if(in.ifNext("!=") || in.ifNext("<>")) {
 	 Operand op(in, tag);
 	 if(isNumber && op.isNumber) {