cortex_m: avoid reading and writing non-existent registers

Change-Id: Iedc24352c8d3444372da06d00fcec9603540f950
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/6059
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

src/target/hla_target.c

@@ -226,7 +226,7 @@ static int adapter_load_context(struct target *target)
 	for (int i = 0; i < num_regs; i++) {
 
 		struct reg *r = &armv7m->arm.core_cache->reg_list[i];
-		if (!r->valid)
+		if (r->exist && !r->valid)
 			armv7m->arm.read_core_reg(target, r, i, ARM_MODE_ANY);
 	}