heap: Fix bug when realloc moves data between heaps

When realloc-ing to a smaller buffer size which ends up allocated in a different heap, the heap
structure is corrupted. This can only happen:

* If heap checking is Comprehensive (meaning buffers are never shrunk in place) and the heap the buffer was originally allocated in is full.
* Calling heap_caps_realloc() to deliberately move a buffer to a different capabilities type, and shrink it at the same time.

Probable fix for https://github.com/espressif/esp-idf/issues/1582
Probably the same issue:
https://www.esp32.com/viewtopic.php?f=2&t=4583
https://www.esp32.com/viewtopic.php?f=13&t=3717

components/heap/heap_caps.c

@@ -309,7 +309,7 @@ IRAM_ATTR void *heap_caps_realloc( void *ptr, size_t size, int caps)
     if (new_p != NULL) {
         size_t old_size = multi_heap_get_allocated_size(heap->heap, ptr);
         assert(old_size > 0);
-        memcpy(new_p, ptr, old_size);
+        memcpy(new_p, ptr, MIN(size, old_size));
         heap_caps_free(ptr);
         return new_p;
     }

components/heap/test/test_realloc.c

@@ -0,0 +1,50 @@
+/*
+ Generic test for realloc
+*/
+
+#include <stdlib.h>
+#include <string.h>
+#include "unity.h"
+#include "sdkconfig.h"
+#include "esp_heap_caps.h"
+
+
+#ifndef CONFIG_HEAP_POISONING_COMPREHENSIVE
+/* (can't realloc in place if comprehensive is enabled) */
+
+TEST_CASE("realloc shrink buffer in place", "[heap]")
+{
+    void *x = malloc(64);
+    TEST_ASSERT(x);
+    void *y = realloc(p, 48);
+    TEST_ASSERT_EQUAL_PTR(x, y);
+}
+
+#endif
+
+TEST_CASE("realloc move data to a new heap type", "[heap]")
+{
+    const char *test = "I am some test content to put in the heap";
+    char buf[64];
+    memset(buf, 0xEE, 64);
+    strlcpy(buf, test, 64);
+
+    char *a = malloc(64);
+    memcpy(a, buf, 64);
+
+    // move data from 'a' to IRAM
+    char *b = heap_caps_realloc(a, 64, MALLOC_CAP_EXEC);
+    TEST_ASSERT_NOT_NULL(b);
+    TEST_ASSERT_NOT_EQUAL(a, b);
+    TEST_ASSERT(heap_caps_check_integrity(MALLOC_CAP_INVALID, true));
+    TEST_ASSERT_EQUAL_HEX32_ARRAY(buf, b, 64/sizeof(uint32_t));
+
+    // Move data back to DRAM
+    char *c = heap_caps_realloc(b, 48, MALLOC_CAP_8BIT);
+    TEST_ASSERT_NOT_NULL(c);
+    TEST_ASSERT_NOT_EQUAL(b, c);
+    TEST_ASSERT(heap_caps_check_integrity(MALLOC_CAP_INVALID, true));
+    TEST_ASSERT_EQUAL_HEX8_ARRAY(buf, c, 48);
+
+    free(c);
+}