Revert "Remove redundant malloc/free"

This reverts commit 6321b1d9c5.

The original commit 6321b1d9c5 switched
from a malloc call of a 22400 byte buffer to a stack-allocated 22400
byte buffer, in addition to the fairly large buffers already allocated
in the functions. Some systems have fairly small stack frames, making
this 22K allocation potentially dangerous. On my stock Debian bullseye
system, I have 200809 bytes (from `getconf
_POSIX_THREAD_ATTR_STACKSIZE`); a 22400 byte buffer already represents
about 10% of the available stacksize.

Other systems, such as those with musl libc, may have either 80KiB or
128KiB [1], making this allocation represent between 18% to 28% of the
available stack space.

[1] https://wiki.musl-libc.org/functional-differences-from-glibc.html#Thread-stack-size

src/crypto/ocb.cc

@@ -1449,11 +1449,13 @@ static void validate()
     ALIGN(16) uint8_t key[32] = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15};
     ALIGN(16) uint8_t valid[] = {0xB2,0xB4,0x1C,0xBF,0x9B,0x05,0x03,0x7D,
                                  0xA7,0xF1,0x6C,0x24,0xA3,0x5C,0x1C,0x94};
-    ALIGN(16) uint8_t val_buf[22400];
     ae_ctx ctx;
-    uint8_t *next = val_buf;
+    uint8_t *val_buf, *next;
     int i, len;
 
+    val_buf = (uint8_t *)malloc(22400 + 16);
+    next = val_buf = (uint8_t *)(((size_t)val_buf + 16) & ~((size_t)15));
+
     if (0) {
 		ae_init(&ctx, key, 16, 12, 16);
 		/* pbuf(&ctx, sizeof(ctx), "CTX: "); */