glibc says memcpy cannot take NULL

At least since 2004, glibc's <string.h> annotates memcpy as
__attribute__((__nonnull__)).  This basedir is passed to it.  When
LOAD_RELATIVE is not defined and MJIT_SEARCH_BUILD_DIR is not set,
this variable is never updated.  Should initialize with meaningful
default value.

mjit.c

@@ -469,7 +469,7 @@ init_header_filename(void)
     // Root path of the running ruby process. Equal to RbConfig::TOPDIR.
     VALUE basedir_val;
 #endif
-    const char *basedir = NULL;
+    const char *basedir = "";
     size_t baselen = 0;
     char *p;
 #ifdef _WIN32