mini-gmp mpz_out_str dereferences null pointer with out-of-range base

[Paul Eggert]

The mpz_out_str implementation in mini-gmp.c computes strlen (NULL) if its base 
argument is out of range. This causes GCC 10.1 -fanalyzer to complain:

mini-gmp.c:4428:9: error: use of NULL 'str' where non-null expected [CWE-690] 
[-Wanalyzer-null-argument]

Proposed patch attached. This patch also fixes an unrelated double-negative in a 
comment that confused me on first reading.
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