Remove possible pointer underflow in fl_utf8_previous_composed_char (#1542)

src/fl_utf8.cxx

@@ -1755,6 +1755,7 @@ const char *fl_utf8_previous_composed_char(const char *from, const char *begin)
       keep = fl_utf8back(from - 1, begin, NULL);
       u = fl_utf8decode(keep, from, NULL);
       if (u == 0x200D) { // zero-width joiner
+        if (keep <= begin) return begin;
         from = fl_utf8back(keep - 1, begin, NULL);
         continue;
       }