Avoid creating intermediate strings in sanitizeMetrics

This patch avoids creating many intermediate strings, when adding dummy width/lsb entries for glyphs where those are missing.
For the relevant PDF files in our test suite, the average number of intermediate strings are well over 1000.
Merge pull request #5219 from Snuffleupagus/sanitizeMetrics-avoid-intermediate-strings

Avoid creating intermediate strings in sanitizeMetrics