fix erroneous assert: cast kBlockSize (of type unsigned int) to "int"

Summary:
Otherwise, we would assert that an unsigned expression is always >= 0.
The intent was to form a possibly negative number, and to assert that
that value is always >= 0, but since one variable in the computation
was unsigned, the result was guaranteed to be unsigned, too, rendering
the assertion useless.

Cast that unsigned variable to "int", so that all operands
are signed, and thus so that the result can be negative.

Test Plan:
  Run "make EXTRA_CXXFLAGS='-W -Wextra'" and see fewer errors.

Reviewers: ljin, sdong, igor.sugak, igor

Reviewed By: igor

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D33771

db/log_writer.cc

@@ -52,7 +52,7 @@ Status Writer::AddRecord(const Slice& slice) {
     }
 
     // Invariant: we never leave < kHeaderSize bytes in a block.
-    assert(kBlockSize - block_offset_ - kHeaderSize >= 0);
+    assert(kBlockSize - block_offset_ >= kHeaderSize);
 
     const size_t avail = kBlockSize - block_offset_ - kHeaderSize;
     const size_t fragment_length = (left < avail) ? left : avail;