Fix potential uninitialized variable (#3602)

Moves a union initialization up a bit so it's performed before code
that can jump to the cleanup target, where file descriptors could
be checked without being initialized.

This could only happen in test code and only in an out-of-memory
situation.

Fixes Coverity 1542254

tools/src/h5perf/sio_engine.c

@@ -139,9 +139,7 @@ do_sio(parameters param, results *res)
     /* IO type */
     iot = param.io_type;
 
-    if (NULL == (fname = calloc(FILENAME_MAX, sizeof(char))))
-        GOTOERROR(FAIL);
-
+    /* MUST initialize fd early since we check its file IDs in cleanup code */
     switch (iot) {
         case POSIXIO:
             fd.posixfd  = -1;
@@ -157,6 +155,9 @@ do_sio(parameters param, results *res)
             GOTOERROR(FAIL);
     }
 
+    if (NULL == (fname = calloc(FILENAME_MAX, sizeof(char))))
+        GOTOERROR(FAIL);
+
     linear_buf_size = 1;
 
     for (i = 0; i < param.rank; i++) {