output_buffer: converted ob_is_empty() to inline

The function ob_is_empty() is called very often.  It's worth it to
convert it to an inline function.

src/outputBuffer.c

@@ -101,11 +101,6 @@ void ob_set_lazy(bool lazy)
 	ob.lazy = lazy;
 }
 
-int ob_is_empty(void)
-{
-	return ob.begin == ob.end;
-}
-
 void ob_shift(void)
 {
 	assert(ob.begin != ob.end);

src/outputBuffer.h

@@ -78,7 +78,10 @@ void ob_flush(void);
 void ob_set_lazy(bool lazy);
 
 /** is the buffer empty? */
-int ob_is_empty(void);
+static inline bool ob_is_empty(void)
+{
+	return ob.begin == ob.end;
+}
 
 void ob_shift(void);