patch: fix segfault against unified diffs which start line is zero

Since 2b1ec74c961f, if a chunk starts with "@@ -0,1", oldstart turns into
a negative value. Because diffhelpers.testhunk() doesn't expect negative bstart,
it bypasses "alen > blen - bstart" condition and segfaults at
"PyList_GET_ITEM(b, i + bstart)".

mercurial/patch.py

@@ -1014,9 +1014,9 b' class hunk(object):'
         oldstart = self.starta + top
         newstart = self.startb + top
         # zero length hunk ranges already have their start decremented
-        if self.lena:
+        if self.lena and oldstart > 0:
             oldstart -= 1
-        if self.lenb:
+        if self.lenb and newstart > 0:
             newstart -= 1
         return old, oldstart, new, newstart
 

tests/test-import.t

@@ -997,6 +997,26 b' import a unified diff with no lines of c'
   c3
   c4
 
+no segfault while importing a unified diff which start line is zero but chunk
+size is non-zero
+
+  $ hg init startlinezero
+  $ cd startlinezero
+  $ echo foo > foo
+  $ hg commit -Amfoo
+  adding foo
+
+  $ hg import --no-commit - << EOF
+  > diff a/foo b/foo
+  > --- a/foo
+  > +++ b/foo
+  > @@ -0,1 +0,1 @@
+  >  foo
+  > EOF
+  applying patch from stdin
+
+  $ cd ..
+
 Test corner case involving fuzz and skew
 
   $ hg init morecornercases