Avoid crash on OOM in virbuftest

The virbuftest code did not check virBufferError before
accessing the buffer contents, resulting in a crash on
OOM conditions.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

tests/virbuftest.c

@@ -108,6 +108,10 @@ static int testBufAutoIndent(const void *data ATTRIBUTE_UNUSED)
     }
     virBufferAdjustIndent(buf, 2);
     virBufferAddLit(buf, "1");
+    if (virBufferError(buf)) {
+        TEST_ERROR("Buffer had error");
+        return -1;
+    }
     if (STRNEQ(virBufferCurrentContent(buf), "  1")) {
         TEST_ERROR("Wrong content");
         ret = -1;
@@ -134,6 +138,11 @@ static int testBufAutoIndent(const void *data ATTRIBUTE_UNUSED)
     virBufferEscapeShell(buf, " 11");
     virBufferAddChar(buf, '\n');
 
+    if (virBufferError(buf)) {
+        TEST_ERROR("Buffer had error");
+        return -1;
+    }
+
     result = virBufferContentAndReset(buf);
     if (!result || STRNEQ(result, expected)) {
         virtTestDifference(stderr, expected, result);
@@ -166,6 +175,11 @@ static int testBufTrim(const void *data ATTRIBUTE_UNUSED)
     virBufferTrim(buf, "b,,", 1);
     virBufferTrim(buf, ",", -1);
 
+    if (virBufferError(buf)) {
+        TEST_ERROR("Buffer had error");
+        return -1;
+    }
+
     result = virBufferContentAndReset(buf);
     if (!result || STRNEQ(result, expected)) {
         virtTestDifference(stderr, expected, result);