tools/coverity: Add model for av_realloc

Really should have done this last time. It should provide consistency
across our allocations and frees.

tools/coverity.c

@@ -58,7 +58,22 @@ void *av_mallocz(size_t size) {
     }
 }
 
+void *av_realloc(void *ptr, size_t size) {
+    int has_memory;
+    __coverity_negative_sink__(size);
+    if (has_memory) {
+        __coverity_escape__(ptr);
+        ptr = __coverity_alloc__(size);
+        __coverity_writeall__(ptr);
+        __coverity_mark_as_afm_allocated__(ptr, "av_free");
+        return ptr;
+    } else {
+        return 0;
+    }
+}
+
 void *av_free(void *ptr) {
     __coverity_free__(ptr);
     __coverity_mark_as_afm_freed__(ptr, "av_free");
 }
+