bdevice.py: Protect _set_pagesize against exceptions.

bdevice.py

@@ -100,6 +100,7 @@ class EepromDevice(BlockDevice):
         if page_size is None:  # Measure it
             self._psize(16)  # Conservative
             old = self[:129]  # Save old contents (nonvolatile!)
+            try:
                 self._psize(256)  # Ambitious
                 r = (16, 32, 64, 128)  # Legal page sizes + 256
                 for x in r:
@@ -111,6 +112,10 @@ class EepromDevice(BlockDevice):
                     ps = 256
                 self._psize(ps)
                 self[:129] = old
+            except:  # If anything goes wrong, restore old data and raise
+                for n, v in enumerate(old):
+                    self[n] = v
+                raise
         else:  # Validated page_size was supplied
             self._psize(page_size)