romio: fix uninitialized variable

Squash compiler warning.

ROMIO is third-party software but has an annoying compiler warning;
this is the minimum distance fix.

Signed-off-by: William Bailey <wbailey2@nd.edu>
(cherry picked from commit 30bda56bcef6f56823ac07f0418fd33e1eff837f)

ompi/mca/io/romio321/romio/adio/common/onesided_aggregation.c

@@ -1966,7 +1966,7 @@ printf("iAmUsedAgg - currentRoundFDStart initialized "
       int *sourceAggBlockLengths=NULL;
       MPI_Aint *sourceAggDisplacements=NULL, *recvBufferDisplacements=NULL;
       MPI_Datatype *sourceAggDataTypes=NULL;
-      char *derivedTypePackedSourceBuffer;
+      char *derivedTypePackedSourceBuffer=NULL;
       int derivedTypePackedSourceBufferOffset = 0;
       int allocatedDerivedTypeArrays = 0;
       ADIO_Offset amountOfDataReadThisRoundAgg = 0;