Invalid time literal causees IllegalArgumentException

[housengw]

minimal example:

target C;
main reactor {
    timer t(0 m)
}

output

wonghouseng@beefy-desktop-linux:~/Desktop/lingua-franca/test/C$ lfc src/exp.lf 
0    [main] ERROR text.validation.CompositeEValidator  - Error executing EValidator
java.lang.IllegalArgumentException
	at org.lflang.JavaAstUtils.toTimeValue(JavaAstUtils.java:160)
	at org.lflang.JavaAstUtils.getLiteralTimeValue(JavaAstUtils.java:213)
	at org.lflang.generator.ReactorInstance.getTimeValue(ReactorInstance.java:635)
	at org.lflang.generator.TimerInstance.<init>(TimerInstance.java:65)
	at org.lflang.generator.ReactorInstance.<init>(ReactorInstance.java:816)

[housengw]

fixed by #916