Fix -Werror=return-type errors.

See also https://bugs.gentoo.org/902483

--- a/converter/other/pngx.c	2023-03-25 02:45:40.000000000 +0200
+++ b/converter/other/pngx.c	2023-03-29 08:34:23.567786499 +0300
@@ -368,6 +368,7 @@
     case PNGX_ABSOLUTE_COLORIMETRIC: return "ABSOLUTE_COLORIMETRIC";
     }
     assert(false);
+    abort();
 }
 
 
@@ -383,6 +384,7 @@
     }
 
     assert(false);  /* All cases above return */
+    abort();
 }
 
 
--- a/editor/pamcat.c	2022-12-31 02:32:37.000000000 +0200
+++ b/editor/pamcat.c	2023-03-29 08:34:45.781144204 +0300
@@ -1211,6 +1211,7 @@
     case JUST_CENTER: return (outpamP->width - inpamP->width) / 2;
     }
     assert(false);
+    abort();
 }
 
 
--- a/lib/libpm.c	2021-12-27 19:33:56.000000000 +0200
+++ b/lib/libpm.c	2023-03-29 08:32:36.504335698 +0300
@@ -92,6 +92,7 @@
 
 
 void
+_Noreturn
 pm_longjmp(void) {
 
     if (pm_jmpbufP)
@@ -259,6 +260,7 @@
 
 
 void PM_GNU_PRINTF_ATTR(1,2)
+_Noreturn
 pm_error(const char format[], ...) {
     va_list args;
     const char * msg;
--- a/lib/libpnm3.c	2022-09-28 05:22:32.000000000 +0300
+++ b/lib/libpnm3.c	2023-03-29 08:33:31.884396460 +0300
@@ -433,6 +433,7 @@
     case PBM_WHITE: return pnm_whitexel(maxval, PBM_TYPE); break;
     default:
         assert(false);
+        abort();
     }
 }