Fix locking region in sjlit_malloc_exec

Cherry-picked from ad89dd8ecd25589d236bd20b36f2abf69f938fd1
(https://github.com/zherczeg/sljit.git)

The locked section needs to extend until we are done modifying internal
block offsets and sizes, otherwise we risk running into data corruption
in a multi-threaded context.

This bug was introduced in 7de0fee9a7376b33e31a3903487cd9933da3a606

Author: lbonn

src/sljit/allocator_src/sljitExecAllocatorCore.c

@@ -237,12 +237,12 @@ SLJIT_API_FUNC_ATTRIBUTE void* sljit_malloc_exec(sljit_uw size)
 		header->size = chunk_size;
 		next_header = AS_BLOCK_HEADER(header, chunk_size);
 	}
-	SLJIT_ALLOCATOR_UNLOCK();
 	next_header->size = 1;
 	next_header->prev_size = chunk_size;
 #ifdef SLJIT_HAS_EXECUTABLE_OFFSET
 	next_header->executable_offset = executable_offset;
 #endif /* SLJIT_HAS_EXECUTABLE_OFFSET */
+	SLJIT_ALLOCATOR_UNLOCK();
 	return MEM_START(header);
 }