bignum: Make bignum_free safer

Signed-off-by: Aris Adamantiadis <aris@0xbadc0de.be>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

include/libssh/libcrypto.h

@@ -67,7 +67,12 @@ typedef BIGNUM*  bignum;
 typedef BN_CTX* bignum_CTX;
 
 #define bignum_new() BN_new()
-#define bignum_free(num) BN_clear_free(num)
+#define bignum_safe_free(num) do { \
+    if ((num) != NULL) { \
+        BN_clear_free((num)); \
+        (num)=NULL; \
+    } \
+    } while(0)
 #define bignum_set_word(bn,n) BN_set_word(bn,n)
 #define bignum_bin2bn(bn,datalen,data) BN_bin2bn(bn,datalen,data)
 #define bignum_bn2dec(num) BN_bn2dec(num)

include/libssh/libgcrypt.h

@@ -61,7 +61,12 @@ int ssh_gcry_dec2bn(bignum *bn, const char *data);
 char *ssh_gcry_bn2dec(bignum bn);
 
 #define bignum_new() gcry_mpi_new(0)
-#define bignum_free(num) gcry_mpi_release(num)
+#define bignum_safe_free(num) do { \
+    if ((num) != NULL) { \
+        gcry_mpi_release((num)); \
+        (num)=NULL; \
+    } \
+    } while (0)
 #define bignum_set_word(bn,n) gcry_mpi_set_ui(bn,n)
 #define bignum_bin2bn(bn,datalen,data) gcry_mpi_scan(data,GCRYMPI_FMT_USG,bn,datalen,NULL)
 #define bignum_bn2dec(num) ssh_gcry_bn2dec(num)

include/libssh/libmbedcrypto.h

@@ -78,7 +78,12 @@ int ssh_mbedcry_rand(bignum rnd, int bits, int top, int bottom);
 int ssh_mbedcry_is_bit_set(bignum num, size_t pos);
 
 #define bignum_new() ssh_mbedcry_bn_new()
-#define bignum_free(num) ssh_mbedcry_bn_free(num);
+#define bignum_safe_free(num) do { \
+    if ((num) != NULL) { \
+        ssh_mbedcry_bn_free(num); \
+        (num)=NULL; \
+    } \
+    } while(0)
 #define bignum_set_word(bn, n) mbedtls_mpi_lset(bn, n) /* TODO fix
                                                           overflow/underflow */
 #define bignum_bin2bn(data, datalen, bn) mbedtls_mpi_read_binary(bn, data, \