Fix for MDEV-4149, backport from 10.0:

  committer: Sergei Golubchik <sergii@pisem.net> 
    fix for a valgrind builds. 
    my_alloca() cannot have MY_THREAD_SPECIFIC, because can be used 
    outside of the THD context. 

include/my_sys.h

@@ -199,7 +199,7 @@ extern void my_large_free(uchar *ptr);
 #define my_alloca(SZ) alloca((size_t) (SZ))
 #define my_afree(PTR) ((void)0)
 #else
-#define my_alloca(SZ) my_malloc(SZ,MYF(MY_FAE|MY_THREAD_SPECIFIC))
+#define my_alloca(SZ) my_malloc(SZ,MYF(MY_FAE))
 #define my_afree(PTR) my_free(PTR)
 #endif /* HAVE_ALLOCA */