Apply modernize-use-nullptr

Setup:
- `mkdir tidy; cd tidy`
- `cmake .. -DCMAKE_BUILD_TYPE=debug -DPROC_TARGET_NUMBER=1 -DCACHE_NAME_SUFFIX=4 -DBINDIR=. -DDATADIR=. -DBUILD_BUNDLE=ON -DWITH_LTO=OFF -DOPTION_OMP=OFF -DCMAKE_EXPORT_COMPILE_COMMANDS=ON`
- `cd ..`
- `find -name '*.cc' -exec clang-tidy-3.8 -header-filter=.* -p=tidy -fix-errors -checks=modernize-use-nullptr {} \;`

rtengine/jpeg_memsrc.cc

@@ -155,7 +155,7 @@ jpeg_memory_src (j_decompress_ptr cinfo, const JOCTET * buffer, size_t bufsize)
      * This makes it unsafe to use this manager and a different source
      * manager serially with the same JPEG object.  Caveat programmer.
      */
-    if (cinfo->src == NULL) { /* first time for this JPEG object? */
+    if (cinfo->src == nullptr) { /* first time for this JPEG object? */
         cinfo->src = (struct jpeg_source_mgr *)
                      (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_PERMANENT,
                              sizeof(my_source_mgr));