Use a global variable protected by a mutex instead of thread-specific-data to record malloc() failures. (CVS 2972)

FossilOrigin-Name: ac090f2ab3b5a792c2fdf897e10060f263e0d408

src/vdbeapi.c

@@ -157,7 +157,7 @@ int sqlite3_step(sqlite3_stmt *pStmt){
   int rc;
 
   /* Assert that malloc() has not failed */
-  assert( !sqlite3ThreadDataReadOnly()->mallocFailed );
+  assert( !sqlite3MallocFailed() );
 
   if( p==0 || p->magic!=VDBE_MAGIC_RUN ){
     return SQLITE_MISUSE;