resolve: do not crash on empty mergestate

I managed to get an empty .hg/merge/state file by interrupting a merge
by pressing Control-C. This lead to this error:

  TypeError: a2b_hex() argument 1 must be string or read-only buffer,
  not None

since localnode is assigned None before the iteration over lines in
the mergestate begins.

mercurial/merge.py

@@ -23,15 +23,13 b' class mergestate(object):'
     def _read(self):
         self._state = {}
         try:
-            localnode = None
             f = self._repo.opener("merge/state")
             for i, l in enumerate(f):
                 if i == 0:
-                    localnode = l[:-1]
+                    self._local = bin(l[:-1])
                 else:
                     bits = l[:-1].split("\0")
                     self._state[bits[0]] = bits[1:]
-            self._local = bin(localnode)
         except IOError, err:
             if err.errno != errno.ENOENT:
                 raise

tests/test-resolve

@@ -25,3 +25,9 b" hg commit -m 'resolved'"
 
 echo % resolve -l, should be empty
 hg resolve -l
+
+# test crashed merge with empty mergestate
+mkdir .hg/merge
+touch .hg/merge/state
+echo % resolve -l, should be empty
+hg resolve -l

tests/test-resolve.out

@@ -6,3 +6,4 b' created new head'
 0 files updated, 0 files merged, 0 files removed, 1 files unresolved
 use 'hg resolve' to retry unresolved file merges or 'hg update -C' to abandon
 % resolve -l, should be empty
+% resolve -l, should be empty