gprs_sm.c: Fix load of misaligned ptr address

Got this Asan runtime error in osmo-sgsn while implementing the MS side of SM:
"""
DMM gprs_sm.c:427 MM(901700000015254/c655d609) -> ACTIVATE PDP CONTEXT REQ: SAPI=3 NSAPI=6 IETF IPv4 /osmo-sgsn/src/sgsn/gprs_sm.c:453:16: runtime error: load of misaligned address 0x61f000064ba3 for type 'uint32_t', which requires 4 byte alignment
0x61f000064ba3: note: pointer points here
 00  06 01 21 00 00 00 00 28  09 27 01 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00
              ^
"""

Change-Id: I8069e3457120b22bb6514fd5d192bff486d7d87f
---
M src/sgsn/gprs_sm.c
1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/src/sgsn/gprs_sm.c b/src/sgsn/gprs_sm.c
index 1194f31..88d1feb 100644
--- a/src/sgsn/gprs_sm.c
+++ b/src/sgsn/gprs_sm.c
@@ -450,7 +450,7 @@
 		DEBUGPC(DMM, "IPv4 ");
 		if (req_pdpa_len >= 6) {
 			struct in_addr ia;
-			ia.s_addr = ntohl(*((uint32_t *) (req_pdpa+2)));
+			ia.s_addr = osmo_load32be(req_pdpa+2);
 			DEBUGPC(DMM, "%s ", inet_ntop(AF_INET, &ia, buf, sizeof(buf)));
 		}
 		break;

