Preprocessor can't match any ELSE or ENDIF statements to the corresponding IFDEF or IFNDEF

Here's the source file:

```
.DEFINE TESTVAR 1 

:START
   SET A, 1
.IFDEF TESTVAR
   SET B, 3
.ENDIF
   SET PC, START
```

Attempting to assemble with dtasm gives this:

```
error: free occuring .ENDIF without a matching .IF near line 7 of 'definetest.dasm16'
assembler: error occurred.
```
