target/cortex_a: fix temporary breakpoint during step

Commit c8926d1457 introduces the
context and hybrid breakpoint types beside existing SW and HW
types. The new field "asid" is non-zero only for these new types.

The commit above did not properly initialize "asid" to 0 for a
temporarily HW breakpoint used during single step. This causes
cortex_a_unset_breakpoint() to identify this breakpoint as of
type "hybrid".

Identified through valgrind error:
Conditional jump or move depends on uninitialised value(s)
Actually valgrind triggers a total of 10 messages about use of
uninitialized variables, but they are all caused by the first
conditional jump bases on "asid != 0".

Fixed by initializing "asid" to 0 in cortex_a_step().

Fixes: c8926d1457 ("cortex_a hybrid & context breakpoints")
Change-Id: Ib674d8457d1e02e10332fd9d73554d13719ef93d
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4613
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>

src/target/cortex_a.c

@@ -1420,6 +1420,7 @@ static int cortex_a_step(struct target *target, int current, target_addr_t addre
 
 	/* Setup single step breakpoint */
 	stepbreakpoint.address = address;
+	stepbreakpoint.asid = 0;
 	stepbreakpoint.length = (arm->core_state == ARM_STATE_THUMB)
 		? 2 : 4;
 	stepbreakpoint.type = BKPT_HARD;