blktrace: fix error handling in merge_blktrace_iologs

If setvbuf() fails, also close merge_fp. This patch fixes the following
Coverity complaint:

*** CID 647237:         Resource leaks  (RESOURCE_LEAK)
/blktrace.c: 811             in merge_blktrace_iologs()
805     	fclose(merge_fp);
806     err_out_file:
807     	free(merge_buf);
808     err_param:
809     	free(bcs);
810
>>>     CID 647237:         Resource leaks  (RESOURCE_LEAK)
>>>     Variable "merge_fp" going out of scope leaks the storage it points to.
811     	return ret;

Fixes: c55d29efc7cb ("blktrace: fix three bugs in merge_blktrace_iologs path")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>