Check the channel packet overhead literal against its expression

CHANNEL_PACKET_OVERHEAD_MAX is a hand-computed copy of
CHANNEL_PACKET_OVERHEAD_SZ, needed because the expression bottoms out in
wolfCrypt enum constants that #if reads as zero. Nothing tied the two
together, so a term added to the expression would leave the #error
guarding DEFAULT_MAX_PACKET_SZ silently ineffective.

Assert the bound in internal.c, where both are ordinary constant
expressions, with a negative-array-size typedef.

Author: ejohnstown

src/internal.c

@@ -98,6 +98,17 @@
 
 #include <wolfssl/wolfcrypt/coding.h>
 
+
+/* CHANNEL_PACKET_OVERHEAD_MAX is the preprocessor's stand-in for
+ * CHANNEL_PACKET_OVERHEAD_SZ, whose terms bottom out in wolfCrypt enum
+ * constants that #if reads as zero. The #error in internal.h therefore
+ * cannot compare the two. Here both are ordinary constant expressions, so
+ * a term added to the expression without bumping the literal fails the
+ * build instead of quietly leaving DEFAULT_MAX_PACKET_SZ unguarded. */
+typedef char wolfSSH_channel_overhead_check[
+        (CHANNEL_PACKET_OVERHEAD_SZ <= CHANNEL_PACKET_OVERHEAD_MAX) ? 1 : -1];
+
+
 /*
 Flags:
   HAVE_WC_ECC_SET_RNG