efi/printf: Fix minor bug in precision handling

A negative precision should be ignored completely, and the presence of a
valid precision should turn off the 0 flag.
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200518190716.751506-10-nivedita@alum.mit.eduSigned-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/libstub/vsprintf.c

@@ -279,9 +279,11 @@ int vsprintf(char *buf, const char *fmt, va_list args)
 				++fmt;
 				/* it's the next argument */
 				precision = va_arg(args, int);
-			}
-			if (precision < 0)
+			} else {
 				precision = 0;
+			}
+			if (precision >= 0)
+				flags &= ~ZEROPAD;
 		}
 
 		/* get the conversion qualifier */