Eliminate unnecessary NULL checks before tjFree()

+ document that tjFree() accepts NULL pointers without complaint.
Effectively, it has had that behavior all along, but the API does not
guarantee that tjFree() will be implemented with free() behind the
scenes, so it's best to formalize the behavior.

tjunittest.c

@@ -550,7 +550,7 @@ static void doTest(int w, int h, const int *formats, int nformats, int subsamp,
 bailout:
   if (chandle) tjDestroy(chandle);
   if (dhandle) tjDestroy(dhandle);
-  if (dstBuf) tjFree(dstBuf);
+  tjFree(dstBuf);
 }
 
 
@@ -666,7 +666,7 @@ static void bufSizeTest(void)
 
 bailout:
   free(srcBuf);
-  if (dstBuf) tjFree(dstBuf);
+  tjFree(dstBuf);
   if (handle) tjDestroy(handle);
 }
 
@@ -839,7 +839,7 @@ static int doBmpTest(const char *ext, int width, int align, int height, int pf,
   unlink(filename);
 
 bailout:
-  if (buf) tjFree(buf);
+  tjFree(buf);
   if (exitStatus < 0) return exitStatus;
   return retval;
 }