Fix uninitialized locals in event/hook handlers from having a value.

Since values for local variables are referenced by offset within a Frame
(not by identifier name), and event/hook handler bodies share a common
Frame, the value offsets for local variables in different handlers may
overlap.  This meant locals in a handler without an initialization may
actually end up referring to the value of a previous handler's local
that has the same Frame offset.  When executing the body, that can
possibly result in a type-conflict error or give give unexpected
results instead of a "use of uninitialized value" error.

This patch makes it so uninitialized locals do always refer to a null
value before executing the body of a event/hook handler, so that using
them without assigning a value within the body will connsistently give
a "use of uninitialized value" error.

Addresses #932.

testing/btest/Baseline/language.uninitialized-local/out

@@ -0,0 +1,2 @@
+error in /Users/jsiwek/Projects/bro/bro/testing/btest/.tmp/language.uninitialized-local/uninitialized-local.bro, line 16: value used but not set (my_string)
+Continuing

testing/btest/language/uninitialized-local.bro

@@ -0,0 +1,23 @@
+# @TEST-EXEC: bro -b %INPUT >out 2>&1
+# @TEST-EXEC: TEST_DIFF_CANONIFIER=$SCRIPTS/diff-remove-abspath btest-diff out
+
+event testit() &priority=10
+	{
+	local my_count: count = 10;
+	}
+
+event testit()
+	{
+	# my_string's value occupies same Frame offset as my_count's from above
+	# handler, but execution of this handler body should still "initialize"
+	# it to a null value instead of referring to a left-over value of my_count.
+	local my_string: string;
+	local my_vector: vector of string;
+	my_vector[0] = my_string;
+	print "Continuing";
+	}
+
+event bro_init()
+	{
+	event testit();
+	}