arch/arm/common: fix build warning on up_allocateheap.c
arch/arm/common: fix build warning on up_allocateheap.c

Actually it's not "be used uninitialized".
Line 169 if statement goes to panic so that line 180 will not be executed.
Other cases set the value for heap_start.
Anyway this commit makes compiler happy.

CC:  common/up_allocateheap.c
/root/tizenrt/os/include/tinyara/arch.h: In function 'up_add_kregion':
common/up_allocateheap.c:180:28: warning: 'heap_start' may be used uninitialized in this function [-Wmaybe-uninitialized]
   heap_size = kregionx_end - heap_start;
               ~~~~~~~~~~~~~^~~~~~~~~~~~
common/up_allocateheap.c:158:8: note: 'heap_start' was declared here
  void *heap_start;
        ^~~~~~~~~~

Signed-off-by: sunghan-chang <c954e25285721d8faa8b1a5c1ae2ef5a35338d32@samsung.com>

jeongchanKim/TizenRT,jeongarmy/TizenRT,junmin-kim/TizenRT,an4967/TizenRT,junmin-kim/TizenRT,sunghan-chang/TizenRT,sunghan-chang/TizenRT,sunghan-chang/TizenRT,jsdosa/TizenRT,Samsung/TizenRT,jeongarmy/TizenRT,an4967/TizenRT,pillip8282/TizenRT,junmin-kim/TizenRT,sunghan-chang/TizenRT,jeongchanKim/TizenRT,jeongarmy/TizenRT,an4967/TizenRT,jeongarmy/TizenRT,pillip8282/TizenRT,Samsung/TizenRT,Samsung/TizenRT,Samsung/TizenRT,jeongarmy/TizenRT,pillip8282/TizenRT,Samsung/TizenRT,jsdosa/TizenRT,an4967/TizenRT,jeongarmy/TizenRT,junmin-kim/TizenRT,pillip8282/TizenRT,junmin-kim/TizenRT,jsdosa/TizenRT,junmin-kim/TizenRT,jeongarmy/TizenRT,an4967/TizenRT,jsdosa/TizenRT,junmin-kim/TizenRT,Samsung/TizenRT,jsdosa/TizenRT,pillip8282/TizenRT,sunghan-chang/TizenRT,an4967/TizenRT,jsdosa/TizenRT,jsdosa/TizenRT,jeongchanKim/TizenRT,jeongchanKim/TizenRT,Samsung/TizenRT,an4967/TizenRT,jeongchanKim/TizenRT,jeongchanKim/TizenRT,pillip8282/TizenRT,jeongchanKim/TizenRT,sunghan-chang/TizenRT,pillip8282/TizenRT,sunghan-chang/TizenRT