Fix an assertion encountered in UntypedMDRVA::Copy().

Fix an assertion where a zero-length buffer was being passed to
UntypedMDRVA::Copy().  This occurred when WriteFile() was given a file whose
size was a multiple of the temporary buffer size.  In this issue's case, the
procfs file "environ" happened to be 2032 bytes, while the temporary buffer
was 1016 bytes.

Patch by Michael Krebs <mkrebs@chromium.org>


git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@792 4c0a9323-5329-0410-9bdc-e9ce6186880e

src/client/linux/minidump_writer/minidump_writer.cc

@@ -1212,6 +1212,15 @@ class MinidumpWriter {
     if (!memory.Allocate(total))
       return false;
     for (MDRVA pos = memory.position(); buffers; buffers = buffers->next) {
+      // Check for special case of a zero-length buffer.  This should only
+      // occur if a file's size happens to be a multiple of the buffer's
+      // size, in which case the final sys_read() will have resulted in
+      // zero bytes being read after the final buffer was just allocated.
+      if (buffers->len == 0) {
+        // This can only occur with final buffer.
+        assert(buffers->next == NULL);
+        continue;
+      }
       memory.Copy(pos, &buffers->data, buffers->len);
       pos += buffers->len;
     }