prevent modifying vector after in-place conversion to string

part of #24388

Author: JeffBezanson

src/array.c

@@ -441,6 +441,11 @@ JL_DLLEXPORT jl_value_t *jl_array_to_string(jl_array_t *a)
         if (jl_is_string(o)) {
             a->flags.isshared = 1;
             *(size_t*)o = jl_array_len(a);
+            a->nrows = 0;
+#ifdef STORE_ARRAY_LEN
+            a->length = 0;
+#endif
+            a->maxsize = 0;
             return o;
         }
     }

test/strings/basic.jl

@@ -810,3 +810,11 @@ let s = "∀x∃y", u = codeunits(s)
     @test_throws ErrorException (u[1] = 0x00)
     @test collect(u) == b"∀x∃y"
 end
+
+# issue #24388
+let v = unsafe_wrap(Vector{UInt8}, "abc")
+    s = String(v)
+    @test_throws BoundsError v[1]
+    push!(v, UInt8('x'))
+    @test s == "abc"
+end