epp: avoid to rely on undefined behavior for fopen.

In the case we didn't define the file to save to, fopen would have
been called with a NULL parameter. The result of that call is not
really defined and could have led to some crash at some point.

src/bin/edje/epp/cpplib.c

@@ -7448,8 +7448,9 @@ extern cpp_options         options;
 void
 using_file(const char *filename, const char type)
 {
-   FILE *f;
+   FILE *f = NULL;
 
+   if (!options.watchfile) return;
    f = fopen(options.watchfile, "a");
    if (!f) return;
    if (anotate)