Fix error check for zlib calls

There are multiple "okay" return values, not just Z_OK. Make sure we
don't bail out needlessly.

common/rdr/ZlibInStream.cxx

@@ -141,7 +141,7 @@ bool ZlibInStream::decompress(bool wait)
     zs->avail_in = bytesIn;
 
   int rc = inflate(zs, Z_SYNC_FLUSH);
-  if (rc != Z_OK) {
+  if (rc < 0) {
     throw Exception("ZlibInStream: inflate failed");
   }
 

common/rdr/ZlibOutStream.cxx

@@ -159,7 +159,7 @@ void ZlibOutStream::deflate(int flush)
 #endif
 
     rc = ::deflate(zs, flush);
-    if (rc != Z_OK) {
+    if (rc < 0) {
       // Silly zlib returns an error if you try to flush something twice
       if ((rc == Z_BUF_ERROR) && (flush != Z_NO_FLUSH))
         break;
@@ -193,7 +193,7 @@ void ZlibOutStream::checkCompressionLevel()
     deflate(Z_SYNC_FLUSH);
 
     rc = deflateParams (zs, newLevel, Z_DEFAULT_STRATEGY);
-    if (rc != Z_OK) {
+    if (rc < 0) {
       // The implicit flush can result in this error, caused by the
       // explicit flush we did above. It should be safe to ignore though
       // as the first flush should have left things in a stable state...