dh-gex: Initialize best_size to make the code mode straight-forward

Coverity thought that the best_nlines could underflow, but the best_size is
initialized to 0 before calling this function so its moot. Adjusting the code
to be better understandable to static analyzers by initializing the variable
inside of the function.

Thanks coverity!

CID 1548873

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/dh-gex.c

@@ -409,6 +409,7 @@ static int ssh_retrieve_dhgroup_file(FILE *moduli,
     size_t line = 0;
     size_t best_nlines = 0;
 
+    *best_size = 0;
     for(;;) {
         line++;
         firstbyte = getc(moduli);