drbg_lib: avoid NULL pointer dereference in drbg_add

Found by Coverity Scan
Reviewed-by: NBernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7511)

(cherry picked from commit 59f90557dd6e35cf72ac72016609d759ac78fcb9)

crypto/rand/drbg_lib.c

@@ -1010,7 +1010,7 @@ static int drbg_add(const void *buf, int num, double randomness)
     int ret = 0;
     RAND_DRBG *drbg = RAND_DRBG_get0_master();
     size_t buflen;
-    size_t seedlen = rand_drbg_seedlen(drbg);
+    size_t seedlen;
 
     if (drbg == NULL)
         return 0;
@@ -1018,6 +1018,8 @@ static int drbg_add(const void *buf, int num, double randomness)
     if (num < 0 || randomness < 0.0)
         return 0;
 
+    seedlen = rand_drbg_seedlen(drbg);
+
     buflen = (size_t)num;
 
     if (buflen < seedlen || randomness < (double) seedlen) {