string: Introduce arr_realloc() and check for integer overflow.

string: Introduce arr_realloc() and check for integer overflow.

Use __builtin_mul_overflow() for the check. This builtin was introduced
in gcc-5, so we need to bump the lowest supported version.

Re-implement para_realloc() as a trivial wrapper for arr_realloc()
to simplify and to avoid duplicating the size check.