Fix null pointer dereference printing an expression.

Parser.parseVariable method should always set the value of the variable
it creates. Failure to do so may end up in the following:

```
$ androidmk  <(printf "FOO:=(X)\nFOO:=bar\n")
parse error:
<input>:3:1: variable already set, previous assignment: FOO@<input>:1:5 = %!s(PANIC=String method: runtime error: invalid memory address or nil pointer dereference) (%!s(PANIC=String method: runtime error: invalid memory address or nil pointer dereference)) false
```

The cause is that calling Parser.Parse to parse `FOO=abc` created
a Variable instance with nil value, causing panic on print attempt.

Test: m androidmk && androidmk  <(printf "FOO:=(X)\nFOO:=bar\n")
(should print:
ERROR:  parse error:
<input>:3:1: variable already set, previous assignment: FOO@<input>:1:5 = X = Not Evaluated (X = Not Evaluated) false)

Change-Id: I296d7984df6d8796e0075f9eb692b234f8c94f08