Fix access beyond end of string when SmcOpenConnection() truncates error message

[dpelle]

Running make test_startup with valgrind, I see this error:

==10232== Memcheck, a memory error detector
==10232== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==10232== Using Valgrind-3.17.0.GIT and LibVEX; rerun with -h for copyright info
==10232== Command: ../vim -f -u NONE --not-a-term --clean -V2Xverbosefile -c set\ verbose?\ verbosefile? -cq
==10232== Parent PID: 10231
==10232== 
==10232== Conditional jump or move depends on uninitialised value(s)
==10232==    at 0x4C332A8: strlen (vg_replace_strmem.c:459)
==10232==    by 0x366557: vim_vsnprintf_typval (message.c:4487)
==10232==    by 0x367DD1: vim_vsnprintf (message.c:4238)
==10232==    by 0x367DD1: vim_snprintf (message.c:4226)
==10232==    by 0x252CF2: xsmp_init (os_unix.c:8075)
==10232==    by 0x13F1D3: main (main.c:323)
==10232==  Uninitialised value was created by a stack allocation
==10232==    at 0x252B72: xsmp_init (os_unix.c:8023)

Error happens in Test_V_file_arg(). Apparently, function...

extern SmcConn SmcOpenConnection (
    char *              /* networkIdsList */,
    SmPointer           /* context */,
    int                 /* xsmpMajorRev */,
    int                 /* xsmpMinorRev */,
    unsigned long       /* mask */,
    SmcCallbacks *      /* callbacks */,
    const char *        /* previousId */,
    char **             /* clientIdRet */,
    int                 /* errorLength */,
    char *              /* errorStringRet */
);

... called in xsmp_init() in os_unix.c:8057 does not NUL
terminate the errorStringRet string argument when the error
string is truncated to errorLength bytes. I did not find
accurate documentation about its behavior when truncation
happens, but the PR fixes the bug anyway.

[codecov]

Codecov Report

Merging #7194 into master will increase coverage by 86.11%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           master    #7194       +/-   ##
===========================================
+ Coverage    2.63%   88.74%   +86.11%     
===========================================
  Files         139      148        +9     
  Lines      150460   162351    +11891     
===========================================
+ Hits         3960   144077   +140117     
+ Misses     146500    18274   -128226     

Impacted Files	Coverage Δ
src/os_unix.c	70.37% <0.00%> (+69.26%)	⬆️
src/if_python.c	83.27% <0.00%> (ø)
src/if_perl.xs	86.08% <0.00%> (ø)
src/if_py_both.h	84.65% <0.00%> (ø)
src/if_tcl.c	90.15% <0.00%> (ø)
src/xxd/xxd.c	75.33% <0.00%> (ø)
src/if_python3.c	87.29% <0.00%> (ø)
src/libvterm/t/harness.c	88.33% <0.00%> (ø)
src/if_ruby.c	91.56% <0.00%> (ø)
src/if_lua.c	93.28% <0.00%> (ø)
... and 135 more

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