Fix load-sampler function

[cyblue9]

When I evaluated following code,

(define fpath  "./libs/piano/SalamanderGrandPianoV2_44.1khz16bit/44.1khz16bit")
(load-sampler piano
              fpath
              0
              parse-salamander-piano)

following error was displayed.

Error: evaluating expr: (load-sampler piano
              fpath
              0
              parse-salamander-piano)
stack-catch: ()
stack-catch: ()
stack-catch: ()
stack-catch: ()
stack-catch: ()
stack-catch: ()
stack-catch: ()
stack-catch: ()
stack-catch: ()
stack-catch: ()
Attempting to return a string from a non-string obj fpath

Maybe, there is a slight mistake in the macro. So, fixed.

Please review!