speller: strip leading whitespace from command, to avoid a sneaky crash

This completes the fix for https://savannah.gnu.org/bugs/?59855.

src/nano.c

@@ -2166,6 +2166,9 @@ int main(int argc, char **argv)
 			free(alt_speller);
 			alt_speller = alt_speller_cmdline;
 		}
+		/* Strip leading whitespace from the speller command, if any. */
+		while (alt_speller && *alt_speller && isblank(*alt_speller))
+			alt_speller++;
 #endif
 
 		/* If an rcfile undid the default settings, copy it to the new flags. */