Minor update for zlib search

cmake/OpenCVFindLibsGrfmt.cmake

@@ -7,11 +7,11 @@ if(BUILD_ZLIB)
   ocv_clear_vars(ZLIB_FOUND)
 else()
   include(FindZLIB)
+  if(ZLIB_VERSION_STRING)
+    #cmake 2.8.2 bug - it fails to determine zlib version
+    unset(ZLIB_VERSION_STRING CACHE)
+  endif()
   if(ZLIB_FOUND)
-    if(ZLIB_VERSION_STRING)
-      #cmake 2.8.2 bug - it fails to determine zlib version
-      unset(ZLIB_VERSION_STRING CACHE)
-    endif()
     ocv_parse_header2(ZLIB "${ZLIB_INCLUDE_DIR}/zlib.h" ZLIB_VERSION "")
   endif()
   if(ZLIB_FOUND AND ANDROID)