lib/base: Avoid compiler warning about use-after-free on Fedora 36

While the local stack pointers could be thought of as "only"
numbers that are not invalidated by the memory they point at
being freed, any use of the pointer after the free is undefined
and so warned about (at best).

gcc version 12.2.1 20220819 (Red Hat 12.2.1-1) (GCC)

Signed-off-by: Andrew Bartlett <abartlet@samba.org>

lib/base/json.c

@@ -715,7 +715,15 @@ parse_string(struct parse_ctx *ctx)
 
         /* Allocate or resize our output buffer if need be */
         if (need || p == pend) {
-            char *tmp = realloc(p0, alloc_len + need + 5 /* slop? */);
+            char *tmp;
+
+            /*
+             * Work out how far p is into p0 to re-esablish p after
+             * the realloc()
+             */
+            size_t p0_to_p_len = (p - p0);
+
+            tmp = realloc(p0, alloc_len + need + 5 /* slop? */);
 
             if (tmp == NULL) {
                 ctx->error = heim_error_create_enomem();
@@ -723,7 +731,12 @@ parse_string(struct parse_ctx *ctx)
                 return NULL;
             }
             alloc_len += need + 5;
-            p = tmp + (p - p0);
+
+            /*
+             * We have two pointers, p and p0, we want to keep them
+             * pointing into the same memory after the realloc()
+             */
+            p = tmp + p0_to_p_len;
             p0 = tmp;
             pend = p0 + alloc_len;
 
@@ -974,8 +987,14 @@ parse_string(struct parse_ctx *ctx)
             free(p0);
             return NULL;
         }
-        p = tmp + (p - p0);
-        pend = tmp + 1 + (pend - p0);
+        /*
+         * We have three pointers, p, pend (which are the same)
+         * and p0, we want to keep them pointing into the same
+         * memory after the realloc()
+         */
+        p = tmp + p0_to_pend_len;
+
+        pend = p + 1;
         p0 = tmp;
     }
     *(p++) = '\0';