Fix remotely exploitable stack overrun in Milliwatt

Milliwatt is vulnerable to a remotely exploitable stack overrun when using
the 'o' option.  This occurs due to the milliwatt_generate function not
accounting for AST_FRIENDLY_OFFSET when calculating the maximum number of
samples it can put in the output buffer.

This patch resolves this issue by taking into account AST_FRIENDLY_OFFSET
when determining the maximum number of samples allowed.  Note that at no
point is remote code execution possible.  The data that is written into the
buffer is the pre-defined Milliwatt data, and not custom data.

(issue ASTERISK-19541)
Reported by: Russell Bryant
Tested by: Matt Jordan
Patches:
  milliwatt_stack_overrun.rev1.txt by Russell Bryant (license 6283)
  Note that this patch was written by Russell, even though Matt uploaded it



git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@359615 65c4cc65-6c06-0410-ace0-fbb531ad65f3

apps/app_milliwatt.c

@@ -77,7 +77,7 @@ static void milliwatt_release(struct ast_channel *chan, void *data)
 static int milliwatt_generate(struct ast_channel *chan, void *data, int len, int samples)
 {
 	unsigned char buf[AST_FRIENDLY_OFFSET + 640];
-	const int maxsamples = sizeof (buf) / sizeof (buf[0]);
+	const int maxsamples = (sizeof (buf) / sizeof (buf[0])) - (AST_FRIENDLY_OFFSET / sizeof(buf[0]));
 	int i, *indexp = (int *) data;
 	struct ast_frame wf = {
 		.frametype = AST_FRAME_VOICE,