pg_regress/pg_isolation_regress: Fix possible nullptr dereference.

[Xing Guo]

Hi hackers,

While playing with pg_regress and pg_isolation_regress, I noticed that
there's a potential nullptr deference in both of them.

How to reproduce:

Specify the `--dbname=` option without providing any database name.

<path>/<to>/pg_regress --dbname=   foo
<path>/<to>/pg_isolation_regress --dbname=   foo

Patch is attached.

-- 
Best Regards,
Xing

diff --git a/src/test/isolation/isolation_main.c 
b/src/test/isolation/isolation_main.c
index 31a0e6b709..a88e9da255 100644
--- a/src/test/isolation/isolation_main.c
+++ b/src/test/isolation/isolation_main.c
@@ -89,7 +89,7 @@ isolation_start_test(const char *testname,
        offset += snprintf(psql_cmd + offset, sizeof(psql_cmd) - offset,
                                           "\"%s\" \"dbname=%s\" < \"%s\" > 
\"%s\" 2>&1",
                                           isolation_exec,
-                                          dblist->str,
+                                          dblist ? dblist->str : "",
                                           infile,
                                           outfile);
        if (offset >= sizeof(psql_cmd))
diff --git a/src/test/regress/pg_regress_main.c 
b/src/test/regress/pg_regress_main.c
index a4b354c9e6..4e089b0f83 100644
--- a/src/test/regress/pg_regress_main.c
+++ b/src/test/regress/pg_regress_main.c
@@ -81,7 +81,7 @@ psql_start_test(const char *testname,
                                           "\"%s%spsql\" -X -a -q -d \"%s\" %s 
< \"%s\" > \"%s\" 2>&1",
                                           bindir ? bindir : "",
                                           bindir ? "/" : "",
-                                          dblist->str,
+                                          dblist ? dblist->str : "",
                                           "-v HIDE_TABLEAM=on -v 
HIDE_TOAST_COMPRESSION=on",
                                           infile,
                                           outfile);