[libc] use vars in string to num fuzz targets

[libc] use vars in string to num fuzz targets

The string to integer and string to float standalone fuzz targets just
ran the functions and didn't do anything with the output. This was
intentional, since they are intended to be used with sanitizers to
detect buffer overflow bugs. Not using the variables was causing compile
warnings, so this patch adds trivial checks to use the variables.

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D144208