Fix segfault if when statement's RHS is unitialized.

If it is ever assigned a value, the body of the when can be triggered as
usual.

Addresses BIT-1176.

src/Trigger.cc

@@ -206,7 +206,7 @@ bool Trigger::Eval()
 		return false;
 		}
 
-	if ( v->IsZero() )
+	if ( ! v || v->IsZero() )
 		{
 		// Not true. Perhaps next time...
 		DBG_LOG(DBG_NOTIFIERS, "%s: trigger condition is false", Name());

testing/btest/Baseline/language.when-unitialized-rhs/out

@@ -0,0 +1,38 @@
+error in /Users/jsiwek/Projects/bro/bro/testing/btest/.tmp/language.when-unitialized-rhs/when-unitialized-rhs.bro, line 9: value used but not set (crashMe)
+error in /Users/jsiwek/Projects/bro/bro/testing/btest/.tmp/language.when-unitialized-rhs/when-unitialized-rhs.bro, line 14: value used but not set (x)
+1
+2
+3
+4
+5
+6
+7
+8
+9
+10
+2nd when stmt executing, 999
+1st when stmt executing, not anymore you don't
+11
+12
+13
+14
+15
+16
+17
+18
+19
+20
+21
+22
+23
+24
+25
+26
+27
+28
+29
+30
+31
+32
+33
+34

testing/btest/language/when-unitialized-rhs.bro

@@ -0,0 +1,32 @@
+# @TEST-EXEC: bro -b -r $TRACES/wikipedia.trace %INPUT >out 2>&1
+# @TEST-EXEC: TEST_DIFF_CANONIFIER=$SCRIPTS/diff-remove-abspath btest-diff out
+
+global crashMe: function(): string;
+global x: int;
+
+event bro_init()
+	{
+	when( local result = crashMe() )
+		{
+		print "1st when stmt executing", result;
+		}
+
+	when( local other_result = x )
+		{
+		print "2nd when stmt executing", other_result;
+		}
+	}
+
+global conn_count = 0;
+
+event new_connection(c: connection)
+	{
+	++conn_count;
+	print conn_count;
+
+	if ( conn_count == 10 )
+		{
+		x = 999;
+		crashMe = function(): string { return "not anymore you don't"; };
+		}
+	}