Check correct variable for NULL

Issue discovered by Coverity scan, CID 43163. This should fix CID 43172 as a
side-effect as well.
Signed-off-by: Austin Seipp <austin@well-typed.com>

rts/Hpc.c

@@ -151,7 +151,7 @@ readTix(void) {
     ws();
     
     lookup = lookupHashTable(moduleHash, (StgWord)tmpModule->modName);
-    if (tmpModule == NULL) {
+    if (lookup == NULL) {
         debugTrace(DEBUG_hpc,"readTix: new HpcModuleInfo for %s",
                    tmpModule->modName);
         insertHashTable(moduleHash, (StgWord)tmpModule->modName, tmpModule);