When environment variable PATH is undefined

[Masamichi HOSODA]

When environment variable PATH is undefined, the following error occurs.

```
C:\tmp\lilypond-2.19.16-0.mingw\$_OUTDIR\usr\bin>set PATH=

C:\tmp\lilypond-2.19.16-0.mingw\$_OUTDIR\usr\bin>lilypond
GNU LilyPond 2.19.16
terminate called after throwing an instance of 'std::logic_error'
  what():  basic_string::_S_construct null not valid

This application has requested the Runtime to terminate it in an unusual way.
Please contact the application's support team for more information.

C:\tmp\lilypond-2.19.16-0.mingw\$_OUTDIR\usr\bin>
```

The following patch can fix this problem.

```
--- a/flower/file-path.cc       2014-03-18 00:29:16.000000000 +0900
+++ b/flower/file-path.cc       2014-12-29 17:18:06.887602900 +0900
@@ -52,6 +52,13 @@
   concat (dirs_, string_split (p, PATHSEP));
 }
 
+void
+File_path::parse_path (const char *p)
+{
+  if (p)
+    parse_path (static_cast<string>(p));
+}
+
 bool
 is_file (const string &file_name)
 {
--- a/flower/include/file-path.hh       2014-03-18 00:29:16.000000000 +0900
+++ b/flower/include/file-path.hh       2014-12-29 17:28:53.070834400 +0900
@@ -44,6 +44,7 @@
   bool try_append (string str);
   void append (const string&);
   void parse_path (const string&);
+  void parse_path (const char*);
   void prepend (const string&);
 };
 
```