preprocessor: Don't access empty lists

libc aborts with:

/usr/include/c++/4.4/debug/list:256:error: attempt to access an element in
    an empty container.

Objects involved in the operation:
sequence "this" @ 0x0xbfffe5f0 {
  type = NSt7__debug4listISsSaISsEEE;
}

otherwise.

lib/preprocessor.cpp

@@ -2131,7 +2131,7 @@ void Preprocessor::handleIncludes(std::string &code, const std::string &filePath
         // filename contains now a file name e.g. "menu.h"
         std::string processedFile;
         std::string filepath;
-        if (headerType == UserHeader)
+        if (headerType == UserHeader && !paths.empty())
             filepath = paths.back();
         std::ifstream fin;
         const bool fileOpened(openHeader(filename, includePaths, filepath, fin));