tests: utils: Tolerate NULL actual data in virTestCompareToFile

The function docs state that 'strcontent' may be NULL. This was added in
8b3a0b28 but that commit neglected to fix rest of the function.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>

tests/testutils.c

@@ -800,8 +800,8 @@ virTestCompareToFile(const char *strcontent,
 
         if (filecontentLen > 0 &&
             filecontent[filecontentLen - 1] == '\n' &&
-            strcontent[strlen(strcontent) - 1] != '\n') {
-            if (virAsprintf(&fixedcontent, "%s\n", strcontent) < 0)
+            cmpcontent[strlen(cmpcontent) - 1] != '\n') {
+            if (virAsprintf(&fixedcontent, "%s\n", cmpcontent) < 0)
                 goto failure;
             cmpcontent = fixedcontent;
         }