Fix missing null-terminator in BLI_string_join_arrayN

Although the documentation says so, the null-terminator was missing.
This could cause crashes when logging shader linking errors as shader
sources are empty in this case.

source/blender/blenlib/intern/string_utils.c

@@ -470,6 +470,7 @@ char *BLI_string_join_arrayN(const char *strings[], uint strings_len)
   for (uint i = 0; i < strings_len; i++) {
     c += BLI_strcpy_rlen(c, strings[i]);
   }
+  *c = '\0';
   return result;
 }