Use p==NULL not !p (in if statements, mainly)

Reviewed-by: Tim Hudson <tjh@openssl.org>

crypto/evp/pmeth_lib.c

@@ -416,7 +416,7 @@ int EVP_PKEY_CTX_ctrl_str(EVP_PKEY_CTX *ctx,
     }
     if (strcmp(name, "digest") == 0) {
         const EVP_MD *md;
-        if (!value || !(md = EVP_get_digestbyname(value))) {
+        if (value == NULL || (md = EVP_get_digestbyname(value)) == NULL) {
             EVPerr(EVP_F_EVP_PKEY_CTX_CTRL_STR, EVP_R_INVALID_DIGEST);
             return 0;
         }