Handle less than 3 observations in ggally_cor

[bk1n]

I encountered the following error when running ggpairs on data with less than 3 overlapping observations between two columns. Also occurs with mappings with less than 3 observations per aesthetic.

Error in cor.test.default(x, y, method = method, use = use) : 
  not enough finite observations

I thought the default behaviour should probably be to handle these by producing NA, or similar, and print a warning, so I implemented in this in ggally_cor()