fix(span): explicitly set span->txt to the return value of lv_mem_rea…

…lloc(lvgl#3005)

* Update lv_span.c

Explicitly set `span->txt` to return of `lv_mem_realloc`

* Update lv_mem.h

Add note about `lv_mem_realloc` return value on failure

src/extra/widgets/span/lv_span.c

@@ -152,7 +152,7 @@ void lv_span_set_text(lv_span_t * span, const char * text)
         span->txt = lv_mem_alloc(strlen(text) + 1);
     }
     else {
-        lv_mem_realloc(span->txt, strlen(text) + 1);
+        span->txt = lv_mem_realloc(span->txt, strlen(text) + 1);
     }
     span->static_flag = 0;
     strcpy(span->txt, text);

src/misc/lv_mem.h

@@ -84,7 +84,7 @@ void lv_mem_free(void * data);
  * @param data pointer to an allocated memory.
  * Its content will be copied to the new memory block and freed
  * @param new_size the desired new size in byte
- * @return pointer to the new memory
+ * @return pointer to the new memory, NULL on failure
  */
 void * lv_mem_realloc(void * data_p, size_t new_size);