Minor comment clarifications and trivial rewrites

cpusupport-ARM-AES.c: the info about _u32 was in a git commit message,
    but it's worth adding a 1-line comment about it.

cpusupport-X86-SSE42*.c: our STYLE says that we should never use
    (unsigned char).  That's what the definition of _mm_crc32_u8() uses,
    but the code in alg/crc32_sse42.c uses (uint8_t), so there's no
    point violating our STYLE by using (unsigned char) here.

network_ssl_compat.c: fix style of assignment & checking.

imalloc.h: split the "return (NULL)" cases from the actual malloc().

libcperciva/cpusupport/Build/cpusupport-ARM-AES.c

@@ -11,10 +11,12 @@ main(void)
 	uint32x4_t lanes = {0};
 	uint8_t arr[16] = {0};
 
+	/* Check AES. */
 	data = vld1q_u8(arr);
 	output = vaeseq_u8(data, key);
 	(void)output; /* UNUSED */
 
+	/* Check _u32: some compilers only support the _u8 variant. */
 	lanes = vdupq_laneq_u32(lanes, 0);
 
 	/* Success! */

libcperciva/util/imalloc.h

@@ -18,15 +18,16 @@ imalloc(size_t nrec, size_t reclen)
 	/* Sanity check. */
 	assert(reclen != 0);
 
+	/* Handle cases where we don't allocate memory. */
 	if (nrec == 0)
 		return (NULL);
-
 	if (nrec > SIZE_MAX / reclen) {
 		errno = ENOMEM;
 		return (NULL);
-	} else {
-		return (malloc(nrec * reclen));
 	}
+
+	/* Allocate memory. */
+	return (malloc(nrec * reclen));
 }
 
 /**