Don't move gcov output to root directory

In scripts/test/coverage, avoid attempting to move gcov output files
to the root directory when given no arguments.  Fixes bug 23741.

changes/bug23741

+  o Minor bugfixes (testing):
+    - Prevent scripts/test/coverage from attempting to move gcov
+      output to the root directory.  Fixes bug 23741; bugfix on
+      0.2.5.1-alpha.

scripts/test/coverage

@@ -29,7 +29,7 @@ for fn in src/or/*.c src/common/*.c; do
           gcov -o $on $fn
           if [ -e $GC ]
           then
-            if [ -n $dst ]
+            if [ -d "$dst" ]
             then
               mv $GC $dst/$GC
             fi