Fix some out of bounds errors caught by the debug build

Author: d-torrance

M2/Macaulay2/d/ctype.d

@@ -67,11 +67,12 @@ ismathoperator(c1:char, c2:char):bool := (
     ismathoperator((int(uchar(c1)) << 8) | int(uchar(c2))));
 
 export isvalidsymbol(s:string):bool := (
-     if !isalpha(s.0) then return false;
-     if ismathoperator(s.0, s.1) && length(s) == utf8charlength(s.0)
+     n := length(s);
+     if n > 0 && !isalpha(s.0) then return false;
+     if n > 1 && ismathoperator(s.0, s.1) && n == utf8charlength(s.0)
      then return true;
-     for i from 0 to length(s) - 1 do (
-	 if !isalnum(s.i) || ismathoperator(s.i, s.(i + 1))
+     for i from 0 to n - 1 do (
+	 if !isalnum(s.i) || (n > i + 1 && ismathoperator(s.i, s.(i + 1)))
 	 then return false);
      true);