virTestCompareToFile: Don't access memory we don't own

After reading the contents of a file some cleanup is performed.
However, the check for it might access a byte outside of the
string - if the file is empty in the first place. Then strlen()
is zero.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

tests/testutils.c

@@ -795,13 +795,17 @@ virTestCompareToFile(const char *strcontent,
     if (virTestLoadFile(filename, &filecontent) < 0 && !virTestGetRegenerate())
         goto failure;
 
-    if (filecontent &&
-        filecontent[strlen(filecontent) - 1] == '\n' &&
+    if (filecontent) {
+        size_t filecontentLen = strlen(filecontent);
+
+        if (filecontentLen > 0 &&
+            filecontent[filecontentLen - 1] == '\n' &&
             strcontent[strlen(strcontent) - 1] != '\n') {
             if (virAsprintf(&fixedcontent, "%s\n", strcontent) < 0)
                 goto failure;
             cmpcontent = fixedcontent;
         }
+    }
 
     if (STRNEQ_NULLABLE(cmpcontent, filecontent)) {
         virTestDifferenceFull(stderr,