[libc] Fix strtod exponent overflow bug

String to float has a condition to prevent overflowing the exponent with
the E notation. To do this it checks if adding that exponent to the
exponent found by parsing the number is greater than the maximum
exponent for the given size of float. The if statements had a gap on
exactly the maximum exponent value that caused it to be treated as the
minimum exponent value. This patch fixes those conditions.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D148152