[perl #120998] avoid caller() crashing on eval '' stack frames

Starting from v5.17.3-150-g19bcb54e caller() on an eval frame would
end up calling Perl_sv_grow() with newlen = 0xFFFFFFFF on 32-bit
systems.

This eventually started segfaulting with v5.19.0-442-gcbcb2a1 which
added code to round up allocations to the nearest 0x100, setting
newlen to 0, faulting when sv_setpvn() attempted to copy its source
string into the zero space provided.