Allow fuzz builds to detect string overruns

If FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION is defined then we don't NUL
terminate ASN1_STRING datatypes. This shouldn't be necessary but we add it
any for safety in normal builds.

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: David Benjamin <davidben@google.com>

crypto/asn1/asn1_lib.c

diff --git a/crypto/asn1/asn1_lib.c b/crypto/asn1/asn1_lib.c
index bdd0ec4..02c34a4 100644
--- a/crypto/asn1/asn1_lib.c
+++ b/crypto/asn1/asn1_lib.c
@@ -301,7 +301,12 @@
     }
     if ((size_t)str->length <= len || str->data == NULL) {
         c = str->data;
+#ifdef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
+        /* No NUL terminator in fuzzing builds */
+        str->data = OPENSSL_realloc(c, len);
+#else
         str->data = OPENSSL_realloc(c, len + 1);
+#endif
         if (str->data == NULL) {
             ERR_raise(ERR_LIB_ASN1, ERR_R_MALLOC_FAILURE);
             str->data = c;
@@ -311,8 +316,13 @@
     str->length = len;
     if (data != NULL) {
         memcpy(str->data, data, len);
-        /* an allowance for strings :-) */
+#ifndef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
+        /*
+         * Add a NUL terminator. This should not be necessary - but we add it as
+         * a safety precaution
+         */
         str->data[len] = '\0';
+#endif
     }
     return 1;
 }