result: avoid dynamically-sized stack allocation

Otherwise, long field names have the potential to cause stack
overflow and perhaps slow down the MRI GC.  This reduces one
extraneous data copy while we're at it, too.

Using the checkstack.pl script in the Linux kernel reveals
no other dynamically sized stack allcations after this change.
usage: objdump -d mysql2.so | ~/linux/scripts/checkstack.pl x86_64
Merge pull request #480 from brianmario/stack-allocation-fix

Avoid dynamically-sized stack allocation