scripts: coredump: fix crashing on non-existent memory region

get_mem_region() may return None for a non-existent memory region
so we need to check first before looking at its properties.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

scripts/coredump/gdbstubs/gdbstub.py

@@ -139,13 +139,14 @@ class GdbStub(abc.ABC):
         barray = b''
         r = get_mem_region(addr)
         while remaining > 0:
-            if addr > r['end']:
-                r = get_mem_region(addr)
-
             if r is None:
                 barray = None
                 break
 
+            if addr > r['end']:
+                r = get_mem_region(addr)
+                continue
+
             offset = addr - r['start']
             barray += r['data'][offset:offset+1]