[FIX] configure.c: Do not assume git

If parsee were downloaded from sources other than git-clone(1) (e.g.: a
tarball), perror(3) fails and/or git(1) is not available on the system,
"git remote get-url origin" would have unexpected results or even return
a NULL pointer, which would cause undefined behaviour on
strchr(repo, ...);

In such conditions, a placeholder string, namely "N/A", is strdup(3)ed
to `repo`.

configure.c

@@ -186,18 +186,34 @@ cmd_stdout(char *cmd)
     FILE *f;
     char *line = NULL;
     size_t size;
+    int result;
     if (!cmd)
     {
-        return NULL;
+        goto failure;
     }
     if (!(f = popen(cmd, "r")))
     {
-        return NULL;
+        goto failure;
     }
 
     getline(&line, &size, f);
-    pclose(f);
+    result = pclose(f);
+
+    if (result < 0)
+    {
+        perror("pclose(3)");
+        goto failure;
+    }
+    else if (result)
+    {
+        fprintf(stderr, "command exited with status %d: %s\n", result, cmd);
+        goto failure;
+    }
     return line;
+
+failure:
+    free(line);
+    return NULL;
 }
 static int
 exec_code(char *program, char *argv[])
@@ -426,10 +442,13 @@ main_build(int argc, char *argv[])
     int opt;
     str_array_t *sources, *images, *utils, *aya;
 
-    if (strchr(repo, '\n'))
+    if (repo)
     {
-        *(strchr(repo,  '\n')) = '\0';
+        char *lf = strchr(repo, '\n');
+        *lf = '\0';
     }
+    else
+        repo = strdup("N/A");
 
     while ((opt = getopt(argc, argv, "sl")) != -1)
     {