fix SIGSEGV caused by redefining realloc() (pxscene#896)

This fixes the following crash:

 Thread 3 "pxscene2dtests" received signal SIGSEGV, Segmentation fault.
 [Switching to Thread 0x7f0703aff700 (LWP 6252)]
 0x00007f070eea114c in __tsan_func_entry () from /lib64/libtsan.so.0
 (gdb) bt
 #0  0x00007f070eea114c in __tsan_func_entry () from /lib64/libtsan.so.0
 pxscene#1  0x00000000005af843 in realloc (ptr=0x0, size=32) at pxCore/tests/pxScene2d/test_imagecache.cpp:43
 pxscene#2  0x00007f070a988645 in pthread_getattr_np (thread_id=<optimized out>, attr=<optimized out>) at pthread_getattr_np.c:176
 pxscene#3  0x00007f070eeba185 in __sanitizer::GetThreadStackTopAndBottom(bool, unsigned long*, unsigned long*) () from /lib64/libtsan.so.0
 pxscene#4  0x00007f070eeba617 in __sanitizer::GetThreadStackAndTls(bool, unsigned long*, unsigned long*, unsigned long*, unsigned long*) () from /lib64/libtsan.so.0
 pxscene#5  0x00007f070eea8be6 in __tsan::ThreadStart(__tsan::ThreadState*, int, unsigned long) () from /lib64/libtsan.so.0
 pxscene#6  0x00007f070ee4b7d6 in __tsan_thread_start_func () from /lib64/libtsan.so.0
 pxscene#7  0x00007f070a98661b in start_thread (arg=0x7f0703aff700) at pthread_create.c:465
 pxscene#8  0x00007f07092db98f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

tests/pxScene2d/test_imagecache.cpp

@@ -26,10 +26,19 @@
 #endif
 
 using namespace std;
-bool failRealloc = false;
 bool defaultCallbackExecuted = false;
 extern void startFileDownloadInBackground(void* data);
 extern bool continueDownloadHandleCheck;
+
+// disabled as it causes crash
+// please note that realloc is also extensively
+// used by various glibc functions/threads
+// also on linux even if the malloc/realloc returns non-null
+// it doesn't guarantee that the returned memory will be available.
+// #define UNAVAILABLE_MEMORY_TEST_ENABLED
+
+#ifdef UNAVAILABLE_MEMORY_TEST_ENABLED // {
+bool failRealloc = false;
 typedef void* (*realloc_t)(void*, size_t);
 
 void* realloc(void *ptr, size_t size)
@@ -48,6 +57,7 @@ void* realloc(void *ptr, size_t size)
   }
   return NULL;
 }
+#endif // } UNAVAILABLE_MEMORY_TEST_ENABLED
 
 class commonTestFns
 {
@@ -701,6 +711,7 @@ class rtHttpCacheTest : public testing::Test, public commonTestFns
      EXPECT_TRUE (RT_ERROR == data.data(contents));
     }
 
+#ifdef UNAVAILABLE_MEMORY_TEST_ENABLED // {
     void memoryUnAvailableTest()
     {
       rtString cacheHeader("");
@@ -716,7 +727,8 @@ class rtHttpCacheTest : public testing::Test, public commonTestFns
       EXPECT_TRUE (false == data.readFileData());
       failRealloc = false;
     }
-
+#endif // } UNAVAILABLE_MEMORY_TEST_ENABLED
+
     private:
       rtString defaultCacheHeader;
 };
@@ -753,7 +765,9 @@ TEST_F(rtHttpCacheTest, httpCacheCompleteTest)
   dataUpdatedAfterEtagTest();
 */
   dataUpdatedAfterEtagDownloadFailedTest();
+#ifdef UNAVAILABLE_MEMORY_TEST_ENABLED // {
   memoryUnAvailableTest();
+#endif // } UNAVAILABLE_MEMORY_TEST_ENABLED
 }
 
 class rtFileDownloaderTest : public testing::Test, public commonTestFns