virConfSaveValue: protect against a NULL pointer reference

Fix xlconfigtest runs build for --enable-test-oom on
        Xen XL-2-XML Parse  channel-pty

Program received signal SIGSEGV, Segmentation fault.

    #0  0x00007ffff3c2b373 in __strchr_sse2 () from /lib64/libc.so.6
==> #1  0x00007ffff7875701 in virConfSaveValue (buf=buf@entry=0x7fffffffd8a0, val=val@entry=0x674750) at util/virconf.c:290
    #2  0x00007ffff7875668 in virConfSaveValue (buf=buf@entry=0x7fffffffd8a0, val=<optimized out>) at util/virconf.c:306
    #3  0x00007ffff78757ef in virConfSaveEntry (buf=buf@entry=0x7fffffffd8a0, cur=cur@entry=0x674780) at util/virconf.c:338
    #4  0x00007ffff78783eb in virConfWriteMem (memory=0x665570 "", len=len@entry=0x7fffffffd910, conf=conf@entry=0x65b940)
        at util/virconf.c:1543
    #5  0x000000000040eccb in testCompareParseXML (replaceVars=<optimized out>, xml=<optimized out>,
        xlcfg=0x662c00 "/home/wtenhave/WORK/libvirt/OOMtesting/libvirt-devel/tests/xlconfigdata/test-channel-pty.cfg")
        at xlconfigtest.c:108
    #6  testCompareHelper (data=<optimized out>) at xlconfigtest.c:205
    #7  0x0000000000410b3a in virTestRun (title=title@entry=0x432cc0 "Xen XL-2-XML Parse  channel-pty",
        body=body@entry=0x40e9b0 <testCompareHelper>, data=data@entry=0x7fffffffd9f0) at testutils.c:247
    #8  0x000000000040f322 in mymain () at xlconfigtest.c:278
    #9  0x0000000000411410 in virTestMain (argc=1, argv=0x7fffffffdba8, func=0x40f660 <mymain>) at testutils.c:992
    #10 0x00007ffff3bc0401 in __libc_start_main () from /lib64/libc.so.6
    #11 0x000000000040e86a in _start ()

    (gdb) frame 1
    #1  0x00007ffff7875701 in virConfSaveValue (buf=buf@entry=0x7fffffffd8a0, val=val@entry=0x674750) at util/virconf.c:290
    290                 if (strchr(val->str, '\n') != NULL) {
    (gdb) print *val
    $1 = {type = VIR_CONF_STRING, next = 0x0, l = 0, str = 0x0, list = 0x0}
Signed-off-by: NWim ten Have <wim.ten.have@oracle.com>
(cherry picked from commit ae5d7582)

src/util/virconf.c

@@ -287,14 +287,16 @@ virConfSaveValue(virBufferPtr buf, virConfValuePtr val)
             virBufferAsprintf(buf, "%llu", val->l);
             break;
         case VIR_CONF_STRING:
-            if (strchr(val->str, '\n') != NULL) {
-                virBufferAsprintf(buf, "\"\"\"%s\"\"\"", val->str);
-            } else if (strchr(val->str, '"') == NULL) {
-                virBufferAsprintf(buf, "\"%s\"", val->str);
-            } else if (strchr(val->str, '\'') == NULL) {
-                virBufferAsprintf(buf, "'%s'", val->str);
-            } else {
-                virBufferAsprintf(buf, "\"\"\"%s\"\"\"", val->str);
+            if (val->str) {
+                if (strchr(val->str, '\n') != NULL) {
+                    virBufferAsprintf(buf, "\"\"\"%s\"\"\"", val->str);
+                } else if (strchr(val->str, '"') == NULL) {
+                    virBufferAsprintf(buf, "\"%s\"", val->str);
+                } else if (strchr(val->str, '\'') == NULL) {
+                    virBufferAsprintf(buf, "'%s'", val->str);
+                } else {
+                    virBufferAsprintf(buf, "\"\"\"%s\"\"\"", val->str);
+                }
             }
             break;
         case VIR_CONF_LIST: {