chan_dahdi: Fix segfault dereferencing a NULL tech_pvt.

The tech support customer was using the AMI Redirect action shortly after
a call was placed.  While the channel tried to do an ast_read(), the
masquerade resulting from the channel redirect took place.  The masquerade
in the middle of the ast_read() resulted in the segfault.

(closes issue AST-1025)
Reported by: Trey Blancher
Patches:
      jira_ast_1025_v1.8_v2.patch (license #5621) patch uploaded by rmudgett
........

Merged revisions 375361 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@375362 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_dahdi.c

@@ -9014,11 +9014,20 @@ static struct ast_frame *dahdi_read(struct ast_channel *ast)
 		CHANNEL_DEADLOCK_AVOIDANCE(ast);
 
 		/*
-		 * For PRI channels, we must refresh the private pointer because
-		 * the call could move to another B channel while the Asterisk
-		 * channel is unlocked.
+		 * Check to see if the channel is still associated with the same
+		 * private structure.  While the Asterisk channel was unlocked
+		 * the following events may have occured:
+		 *
+		 * 1) A masquerade may have associated the channel with another
+		 * technology or private structure.
+		 *
+		 * 2) For PRI calls, call signaling could change the channel
+		 * association to another B channel (private structure).
 		 */
-		p = ast->tech_pvt;
+		if (ast->tech_pvt != p) {
+			/* The channel is no longer associated.  Quit gracefully. */
+			return &ast_null_frame;
+		}
 	}
 
 	idx = dahdi_get_index(ast, p, 0);