Prevent an implicit int promotion in malloc/tst-alloc_buffer.c

According to ISO C11, section 6.5.3.3 "Unary arithmetic operators", the
result of the ~ operator is the bitwise complement of its (promoted)
operand.
This can lead to a comparison of a char with another integer type.

Tested on powerpc, powerpc64 and powerpc64le.

	* malloc/tst-alloc_buffer.c (test_misaligned): Cast to char
	before comparing with another char.

malloc/tst-alloc_buffer.c

@@ -429,7 +429,7 @@ test_misaligned (char pad)
   }
 
   /* Verify that padding was not overwritten.  */
-  TEST_VERIFY (backing[0] == ~pad);
+  TEST_VERIFY (backing[0] == (char) ~pad);
   TEST_VERIFY (backing[SIZE + 1] == pad);
   free (backing);
 }