Describe the bug
When trying to compute assortativity for categorical labels attached to nodes, an error is returned when the labels are letters but not when the labels are numbers. It is mentioned that the bug is unexpected and to report it with a reproducible example.

To reproduce

The script below reproduces the bug

library(igraph)

set.seed(2)
g <- sample_gnm(10, 20)

V(g)$random1 <- sample(c(1, 2), 10, replace = T)
V(g)$random2 <- sample(c('1', '2'), 10, replace = T)
V(g)$random3 <- sample(c('A', 'B'), 10, replace = T)

# compute the assortativity of this node attribute 
assortativity_nominal(g, types = V(g)$random1) # this is OK 
assortativity_nominal(g, types = V(g)$random2) # this is OK
assortativity_nominal(g, types = V(g)$random3) # this leads to the output below

Error in assortativity_nominal(g, types = V(g)$random3) :
At core/core/vector.pmt:126 : Assertion failed: size >= 0. This is an unexpected igraph error; please report this as a bug, along with the steps to reproduce it.
Please restart your R session to avoid crashes or other surprising behavior.
In addition: Warning message:
In assortativity_nominal(g, types = V(g)$random3) :
NAs introduced by coercion

Version information
Which version of igraph are you using and where did you obtain it?

igraph_1.6.0
from CRAN

R version 4.3.3 (2024-02-29)
Platform: x86_64-pc-linux-gnu (64-bit)