Fix incorrect buffer pool adjustment during sampler start.

Author: jovanstevanovic

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/jfr/sampler/AbstractJfrExecutionSampler.java

@@ -262,8 +262,8 @@ protected void operate() {
             boolean shouldSample = shouldSample();
             if (sampler.isSampling != shouldSample) {
                 if (shouldSample) {
-                    sampler.startSampling();
                     sampler.isSampling = true;
+                    sampler.startSampling();
                 } else {
                     sampler.stopSampling();
                     sampler.isSampling = false;