Fixed invalid array access on EXS_Unknown.

Fixed invalid access to XferNames array for transfer syntax enum
EXS_Unknown (-1). This issue was introduced only recently with commit
9872714.

Thanks to GitHub user "percontation" for the report and suggested fix.
This closes GitHub pull request #107.

dcmdata/libsrc/dcxfer.cc

@@ -853,8 +853,8 @@ DcmXfer::DcmXfer(E_TransferSyntax xfer)
 {
     /* casting the enum to an integer should be safe */
     const int i = OFstatic_cast(int, xfer);
-    /* ... but you never know, so double-check this */
-    if ((i < DIM_OF_XferNames) && (XferNames[i].xfer == xfer))
+    /* ... but double-check this to avoid invalid array access, e.g. for EXS_Unknown */
+    if ((i >= 0) && (i < DIM_OF_XferNames) && (XferNames[i].xfer == xfer))
     {
         xferSyn               = XferNames[i].xfer;
         xferID                = XferNames[i].xferID;
@@ -959,8 +959,8 @@ DcmXfer &DcmXfer::operator=(const E_TransferSyntax xfer)
 {
     /* casting the enum to an integer should be safe */
     const int i = OFstatic_cast(int, xfer);
-    /* ... but you never know, so double-check this  */
-    if ((i < DIM_OF_XferNames) && (XferNames[i].xfer == xfer))
+    /* ... but double-check this to avoid invalid array access, e.g. for EXS_Unknown */
+    if ((i >= 0) && (i < DIM_OF_XferNames) && (XferNames[i].xfer == xfer))
     {
         xferSyn               = XferNames[i].xfer;
         xferID                = XferNames[i].xferID;