Fix signature-related memory leaks.

ECDSA_SIG_feee() wasn't called in two places.
Avoid generating too short keys.

Rarely, the ECDSA signature was one byte shorted than expected; this is
due to zero-leading bignum in one of the components.  Because the rest
of the code, including validation, can't deal with such (valid)
signatures, just discard them and repeat until a well-formatted key is
produced.
Fix one-byte-too-small buffer in elgen.
Include terminating NULL in el_base32_decode_buffer_size().

Return buffer size *including* the terminating NULL character.  This is
safer for use (case in point: commit
4e439753b3cbddf9a6c87918901e696c6deb7fd0).