eel-string: avoid 'memcpy' with NULL argument

Fixes Clang static analyzer warning:

eel-string.c:325:9: warning: Null pointer passed as an argument to a 'nonnull' parameter
        memcpy (result_position, replacement, replacement_length);
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

1 files changed, 4 insertions, 0 deletions

diff --git a/eel/eel-string.c b/eel/eel-string.c
index c44706c1..c71aadb7 100644
--- a/eel/eel-string.c
+++ b/eel/eel-string.c
@@ -292,6 +292,10 @@ eel_str_replace_substring (const char *string,
     {
         return NULL;
     }
+    else if (replacement == NULL)
+    {
+        return (char *) string;
+    }
 
     substring_length = strlen (substring);
     replacement_length = eel_strlen (replacement);