tax_glom: Add initial check for NAs in the otu_table

[mikemc]

Otherwise can fail with cryptic message; See https://github.com/joey711/phyloseq/issues/1110 (the message is different for speedyseq's tax_glom, but similarly unhelpful):

Error in `taxa_names<-`(`*tmp*`, value = gsub("\"", "", taxa_names(x),  :
taxa_names<-: You are attempting to assign duplicated taxa_names