Cast PTRDIFF_MAX to size_t before adding 1.

This fixes compilation warnings regarding integer overflow that were
introduced by 0c516a00c4cb28cff55ce0995f756b5aae074c9e (Make *allocx()
size class overflow behavior defined.).
diff --git a/test/integration/mallocx.c b/test/integration/mallocx.c
index 35c559a..6ecd636 100644
--- a/test/integration/mallocx.c
+++ b/test/integration/mallocx.c
@@ -55,8 +55,8 @@
 	assert_ptr_null(mallocx(hugemax+1, 0),
 	    "Expected OOM for mallocx(size=%#zx, 0)", hugemax+1);
 
-	assert_ptr_null(mallocx(PTRDIFF_MAX+1, 0),
-	    "Expected OOM for mallocx(size=%#zx, 0)", ZU(PTRDIFF_MAX+1));
+	assert_ptr_null(mallocx(ZU(PTRDIFF_MAX)+1, 0),
+	    "Expected OOM for mallocx(size=%#zx, 0)", ZU(PTRDIFF_MAX)+1);
 
 	assert_ptr_null(mallocx(SIZE_T_MAX, 0),
 	    "Expected OOM for mallocx(size=%#zx, 0)", SIZE_T_MAX);
@@ -69,9 +69,9 @@
 	assert_ptr_null(mallocx(size, 0),
 	    "Expected OOM for mallocx(size=%#zx, 0", size);
 
-	assert_ptr_null(mallocx(1, MALLOCX_ALIGN(PTRDIFF_MAX+1)),
+	assert_ptr_null(mallocx(1, MALLOCX_ALIGN(ZU(PTRDIFF_MAX)+1)),
 	    "Expected OOM for mallocx(size=1, MALLOCX_ALIGN(%#zx))",
-	    ZU(PTRDIFF_MAX+1));
+	    ZU(PTRDIFF_MAX)+1);
 }
 TEST_END
 
diff --git a/test/integration/rallocx.c b/test/integration/rallocx.c
index 3b7d21c..c3c2241 100644
--- a/test/integration/rallocx.c
+++ b/test/integration/rallocx.c
@@ -232,8 +232,8 @@
 	assert_ptr_null(rallocx(p, hugemax+1, 0),
 	    "Expected OOM for rallocx(p, size=%#zx, 0)", hugemax+1);
 
-	assert_ptr_null(rallocx(p, PTRDIFF_MAX+1, 0),
-	    "Expected OOM for rallocx(p, size=%#zx, 0)", ZU(PTRDIFF_MAX+1));
+	assert_ptr_null(rallocx(p, ZU(PTRDIFF_MAX)+1, 0),
+	    "Expected OOM for rallocx(p, size=%#zx, 0)", ZU(PTRDIFF_MAX)+1);
 
 	assert_ptr_null(rallocx(p, SIZE_T_MAX, 0),
 	    "Expected OOM for rallocx(p, size=%#zx, 0)", SIZE_T_MAX);
@@ -246,9 +246,9 @@
 	assert_ptr_null(rallocx(p, size, 0),
 	    "Expected OOM for rallocx(p, size=%#zx, 0", size);
 
-	assert_ptr_null(rallocx(p, 1, MALLOCX_ALIGN(PTRDIFF_MAX+1)),
+	assert_ptr_null(rallocx(p, 1, MALLOCX_ALIGN(ZU(PTRDIFF_MAX)+1)),
 	    "Expected OOM for rallocx(p, size=1, MALLOCX_ALIGN(%#zx))",
-	    ZU(PTRDIFF_MAX+1));
+	    ZU(PTRDIFF_MAX)+1);
 
 	dallocx(p, 0);
 }
diff --git a/test/unit/size_classes.c b/test/unit/size_classes.c
index 3a2126f..2e2caaf 100644
--- a/test/unit/size_classes.c
+++ b/test/unit/size_classes.c
@@ -88,14 +88,14 @@
 
 	assert_u_ge(size2index(max_size_class+1), NSIZES,
 	    "size2index() should return >= NSIZES on overflow");
-	assert_u_ge(size2index(PTRDIFF_MAX+1), NSIZES,
+	assert_u_ge(size2index(ZU(PTRDIFF_MAX)+1), NSIZES,
 	    "size2index() should return >= NSIZES on overflow");
 	assert_u_ge(size2index(SIZE_T_MAX), NSIZES,
 	    "size2index() should return >= NSIZES on overflow");
 
 	assert_zu_gt(s2u(max_size_class+1), HUGE_MAXCLASS,
 	    "s2u() should return > HUGE_MAXCLASS for unsupported size");
-	assert_zu_gt(s2u(PTRDIFF_MAX+1), HUGE_MAXCLASS,
+	assert_zu_gt(s2u(ZU(PTRDIFF_MAX)+1), HUGE_MAXCLASS,
 	    "s2u() should return > HUGE_MAXCLASS for unsupported size");
 	assert_zu_eq(s2u(SIZE_T_MAX), 0,
 	    "s2u() should return 0 on overflow");