Fix #81070: Integer underflow when memory limit is exceeded

When the memory limit is reduced using an `ini_set("memory_limit", ..)`
below the currently allocated memory. Further allocations should
cause a memory limit exceeded error.
This fixes the regression introduced in 7.2.23

Author: pvandommelen

Zend/tests/bug81070.phpt

@@ -0,0 +1,10 @@
+--TEST--
+Bug #81070	Integer underflow in memory limit comparison
+--FILE--
+<?php
+$a = str_repeat("0", 5 * 1024 * 1024);
+ini_set("memory_limit", "3M");
+$b = str_repeat("0", 5 * 1024 * 1024);
+?>
+--EXPECTF--
+Fatal error: Allowed memory size of 3145728 bytes exhausted%s(tried to allocate %d bytes) in %s on line %d

Zend/zend_alloc.c

@@ -948,7 +948,7 @@ static void *zend_mm_alloc_pages(zend_mm_heap *heap, uint32_t pages_count ZEND_F
 				heap->cached_chunks = chunk->next;
 			} else {
 #if ZEND_MM_LIMIT
-				if (UNEXPECTED(ZEND_MM_CHUNK_SIZE > heap->limit - heap->real_size)) {
+				if (UNEXPECTED(heap->limit < heap->real_size || ZEND_MM_CHUNK_SIZE > heap->limit - heap->real_size)) {
 					if (zend_mm_gc(heap)) {
 						goto get_chunk;
 					} else if (heap->overflow == 0) {
@@ -1474,8 +1474,8 @@ static zend_never_inline void *zend_mm_realloc_huge(zend_mm_heap *heap, void *pt
 			}
 		} else /* if (new_size > old_size) */ {
 #if ZEND_MM_LIMIT
-			if (UNEXPECTED(new_size - old_size > heap->limit - heap->real_size)) {
-				if (zend_mm_gc(heap) && new_size - old_size <= heap->limit - heap->real_size) {
+			if (UNEXPECTED(heap->limit < heap->real_size || new_size - old_size > heap->limit - heap->real_size)) {
+				if (zend_mm_gc(heap) && heap->limit >= heap->real_size && new_size - old_size <= heap->limit - heap->real_size) {
 					/* pass */
 				} else if (heap->overflow == 0) {
 #if ZEND_DEBUG
@@ -1766,8 +1766,8 @@ static void *zend_mm_alloc_huge(zend_mm_heap *heap, size_t size ZEND_FILE_LINE_D
 	}
 
 #if ZEND_MM_LIMIT
-	if (UNEXPECTED(new_size > heap->limit - heap->real_size)) {
-		if (zend_mm_gc(heap) && new_size <= heap->limit - heap->real_size) {
+	if (UNEXPECTED(heap->limit < heap->real_size || new_size > heap->limit - heap->real_size)) {
+		if (zend_mm_gc(heap) && heap->limit >= heap->real_size && new_size <= heap->limit - heap->real_size) {
 			/* pass */
 		} else if (heap->overflow == 0) {
 #if ZEND_DEBUG