Add fallback for sysconf(_SC_GETPW_R_SIZE_MAX)

According to getpwnam(3), sysconf(_SC_GETPW_R_SIZE_MAX) can return
-1 and does so on musl.  Fall back to a reasonable value that
matches what glibc returns.

Bug: 190084016
Test: m USE_HOST_MUSL=true host-native -k
Change-Id: I373e962cee5e85eeedbdc2c0e5fd180fa5cb8b6d
1 file changed