psql: Fix %w length in PROMPT2 when PROMPT1 contains a newline.

The width of the invisible PROMPT2 must take into account, in order
for user input to be aligned with the first line, that PROMPT1 can
contain newlines.

Author: Maxence Ahlouche
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/CAJeaomVyLSP_Wj%3D0FtYNTuoopWHyFarhUtYKDHs0HHv%2Bb%3DN9sA%40mail.gmail.com

src/bin/psql/prompt.c

@@ -373,7 +373,10 @@ get_prompt(promptStatus_t status, ConditionalStack cstack)
 				if (visible)
 				{
 					chwidth = PQdsplen(p, pset.encoding);
-					if (chwidth > 0)
+
+					if (*p == '\n')
+						last_prompt1_width = 0;
+					else if (chwidth > 0)
 						last_prompt1_width += chwidth;
 				}