filenamecat-lgpl: set errno upon failure

[Bruno Haible]

POSIX <https://pubs.opengroup.org/onlinepubs/9699919799/functions/opendir.html>
says that when opendir() fails, it return NULL with errno set.

To implement this in gnulib for native Windows,
  - the function _gl_register_fd must set errno upon failure,
  - therefore get_name must set errno upon failure,
  - therefore mfile_name_concat must set errno upon failure.


2020-06-26  Bruno Haible  <bruno@clisp.org>

        filenamecat-lgpl: Set errno upon failure.
        * lib/filenamecat-lgpl.c (mfile_name_concat): Document the failure
        return convention.
        * modules/filenamecat-lgpl (Depends-on): Add malloc-posix.

diff --git a/lib/filenamecat-lgpl.c b/lib/filenamecat-lgpl.c
index 6f666f2..d97abfa 100644
--- a/lib/filenamecat-lgpl.c
+++ b/lib/filenamecat-lgpl.c
@@ -41,7 +41,7 @@
    *BASE_IN_RESULT to point to the copy of BASE at the end of the
    returned concatenation.
 
-   Return NULL if malloc fails.  */
+   If malloc fails, return NULL with errno set.  */
 
 char *
 mfile_name_concat (char const *dir, char const *base, char **base_in_result)
@@ -68,20 +68,22 @@ mfile_name_concat (char const *dir, char const *base, char 
**base_in_result)
     }
 
   char *p_concat = malloc (dirlen + (sep != '\0')  + baselen + 1);
-  char *p;
-
   if (p_concat == NULL)
     return NULL;
 
-  p = mempcpy (p_concat, dir, dirlen);
-  *p = sep;
-  p += sep != '\0';
+  {
+    char *p;
+
+    p = mempcpy (p_concat, dir, dirlen);
+    *p = sep;
+    p += sep != '\0';
 
-  if (base_in_result)
-    *base_in_result = p;
+    if (base_in_result)
+      *base_in_result = p;
 
-  p = mempcpy (p, base, baselen);
-  *p = '\0';
+    p = mempcpy (p, base, baselen);
+    *p = '\0';
+  }
 
   return p_concat;
 }
diff --git a/modules/filenamecat-lgpl b/modules/filenamecat-lgpl
index 09965a1..a6bf60b 100644
--- a/modules/filenamecat-lgpl
+++ b/modules/filenamecat-lgpl
@@ -9,6 +9,7 @@ m4/filenamecat.m4
 Depends-on:
 c99
 dirname-lgpl
+malloc-posix
 
 configure.ac:
 gl_FILE_NAME_CONCAT_LGPL