Fix length of ptx when compilation succeeded

Author: lamblin

src/gpuarray_buffer_cuda.c

@@ -1116,10 +1116,10 @@ static int call_compiler(cuda_context *ctx, strb *src, strb *ptx, strb *log) {
   if (strb_ensure(ptx, buflen) == 0) {
     err = nvrtcGetPTX(prog, ptx->s+ptx->l);
     if (err != NVRTC_SUCCESS) {
-      ptx->l += buflen;
       nvrtcDestroyProgram(&prog);
       return error_nvrtc(ctx->err, "nvrtcGetPTX", err);
     }
+    ptx->l += buflen;
   }
 
   return GA_NO_ERROR;